Voters and election officials were in some cases navigating challenging conditions, including the fallout from natural disasters. Poll workers set up a temporary tent polling station in Yancey County, N.C., where communities are still recovering from Hurricane Helene. In Asheville, N.C., officials sent voters from two precincts to a center southeast of the city after hurricane damage compromised access to their usual polling place.

Whether Americans turn to Kamala Harris, the Democratic vice president, or Donald J. Trump, the Republican former president, voters will be making history: Ms. Harris would be the first woman elected to the White House; Mr. Trump would be the first convicted felon to sit in the Oval Office.

This Election Day brings to an end an emotionally grueling campaign that has epitomized all the dysfunction and polarization in American politics. To the end, nearly every poll in the seven battleground states showed the race to be a tossup.

“We need everyone to vote, Pennsylvania, you will make the difference in this election,” Ms. Harris said in her final pitch on Monday, after a five-city swing across Pennsylvania. “You will make the difference.”

Mr. Trump, addressing supporters in Pittsburgh on Monday night, dismissed polls showing a close race as he urged his voters to turn out. “Let everyone think it’s going to be tight as hell,” he said. “Whether it is or not, you’ve got to go out and you have to swamp them.”

No less perplexing is when the victor will become clear. If the polls are correct, and the race is this tight, it could be days before the next president is known. If the polls are wrong, the nation could awake Wednesday morning knowing who will succeed President Biden in the White House.

Mr. Trump is planning to spend Election Day in Florida, where he will vote before returning to his home in Mar-a-Lago, his residence in Palm Beach. Ms. Harris will be working out of the Naval Observatory, the vice president’s residence in Washington, where aides said she would do interviews with radio stations in all seven battleground states.

It is, with all its tension and uncertainty, a fitting conclusion for an extraordinary contest that was marked by two assassination attempts on Mr. Trump, and Mr. Biden’s withdrawal from the race in July.

Here’s what else to know:

U.S. warns of foreign election interference: U.S. intelligence and law enforcement officials are warning that Russia, and to a lesser extent Iran, will most likely flood social media with misinformation on Election Day and for weeks afterward in an effort to subvert confidence in the presidential election.

Jewish voters on edge: The intensity of the political season has compounded anxieties in Jewish communities across the United States. Jewish voters in the hotly contested battleground state of Pennsylvania are being courted by the leaders of both political parties while being repelled by elements in each of them.

Rogan endorses Trump: Joe Rogan, the enormously popular podcast host who brought Mr. Trump, his running mate JD Vance, and Elon Musk onto his show in recent weeks, endorsed the former president in a post on social media.

When will we know?: Counting the votes will extend beyond election night on Tuesday, and determining the outcome of the presidential race could, too. Here’s a look at how votes are counted in key states — Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin — and when to expect results.

Working-class white voters

Macomb County, Mich.

Macomb County, a stretch of suburbs and exurbs north of Detroit, is home to large numbers of the working-class white voters who broke the so-called Blue Wall in 2016, flipping Michigan, Pennsylvania and Wisconsin to Republicans for the first time in more than a quarter-century and winning Mr. Trump the presidency.

Four years later, just enough of them returned to vote Democratic, helping Joseph R. Biden Jr. flip those states back. How those voters break this year could be decisive again.

In 2020, Mr. Trump won 53.4 percent of the vote in Macomb County.

College-educated suburban voters

Chester County, Pa.

Chester County, an affluent area west of Philadelphia, is filled with the sort of highly educated suburban voters who have shifted toward voting for Democrats since 2016, one of the more electorally significant realignments of the Trump era.

Educated suburban voters, especially women, helped fuel Democratic victories in 2018, 2020 and 2022. Abortion has been a motivating issue for suburban women, and if we see a big “Dobbs effect” of backlash to the overturning of Roe v. Wade, this is one place where it could be visible.

In 2020, Mr. Biden won 57.8 percent of the vote in Chester County.

Black voters

Fulton County, Ga.

Black voters are a powerful bloc in Fulton County, home to Atlanta, and Mr. Trump has been trying to cut into the Democrats’ advantage there.

Democrats have historically relied on huge margins in heavily Black metropolitan areas in order to win states that, on a traditional results map, appear mostly red because of strong Republican support in geographically large but less populated areas. This was a big part of what enabled Mr. Biden to win Georgia, Michigan, Pennsylvania and Wisconsin.

In 2020, Mr. Biden won 72.6 percent of the vote in Fulton County.

Latino voters

Yuma County, Ariz.

Yuma County, in the southwestern corner of Arizona along the border with Mexico, is about two-thirds Latino and has been at the center of debates over the effects of illegal immigration. How it votes will offer some indication of how Latino voters are weighing that issue, though it’s important to note that there is also a sizable minority of white working-class voters here.

Another word of caution: It’s tough to identify bellwethers for Latino voters at large, in part because Latinos are so diverse. Cuban Americans in Florida, for instance, are politically distinct from Mexican Americans in Arizona.

In 2020, Mr. Trump won 52.2 percent of the vote in Yuma County.

Arab American voters

Wayne County, Mich.

Wayne County is home to Detroit, a heavily Black city, but it’s also home to Dearborn, a majority-Arab suburb that has been an epicenter of anger at the Biden administration’s and Harris campaign’s support for Israel in its war against Hamas.

Many voters here have historically backed Democrats but are disillusioned with Ms. Harris because she says she would continue to provide weapons to Israel. Some have said they will reluctantly vote for her anyway, but others plan to vote for Mr. Trump or a third-party candidate, or not to vote at all. Victory margins and turnout numbers will both be significant.

In 2020, Mr. Biden won 68.4 percent of the vote in Wayne County.

Jewish voters

Montgomery County, Pa.

Montgomery County is home to significant Jewish populations, particularly in Lower Merion Township, west of Philadelphia. It is an imperfect bellwether — no county is majority Jewish, and many places with the largest Jewish minorities, like New York City, aren’t in battleground states — but it will help show another piece of the electoral impact of Israel’s war in Gaza.

Mr. Trump has been trying to woo Jewish voters, saying his support of Israel would be uncompromising compared with that of Ms. Harris, who has sometimes criticized it. In doing so, though, he has often used antisemitic tropes.

In 2020, Mr. Biden won 62.6 percent of the vote in Montgomery County.

Young voters

Dane County, Wis.

Dane County is home to the University of Wisconsin-Madison, and like other counties with large college campuses, it will point to trends among young people, including first-time voters.

Young voters have historically been an important constituency for Democrats, and Ms. Harris has drawn more enthusiasm from them than Mr. Biden was drawing earlier this year. But there are still questions surrounding her support, especially because of Israel, and there is evidence of a growing gender gap caused by young men moving to the right.

In 2020, Mr. Biden won 75.5 percent of the vote in Dane County.

Rural voters

Peach County, Ga.

It’s hard to choose one bellwether county for rural voters because they are spread across such a vast number of counties, most of which are heavily Republican.

But one option is Peach County, a swath of central Georgia that — unlike most rural counties — is closely split between white and Black voters. It has swung back and forth between Republicans and Democrats in past elections.

In 2020, Mr. Trump won 51.8 percent of the vote in Peach County.

The question remains if the presidential election will be so close that the court, which has a 6-3 conservative majority, will take up a case in the days or weeks ahead that decides who the next president is.

It is unlikely that the Supreme Court ends up playing a major role in the outcome, according to elections experts, but it is possible. Here’s what to know.

What role could the Supreme Court play?

The Supreme Court has generally tried to stay out of political and electoral fights, and most election-related litigation will remain in the lower courts. But once a case is in the court system, it becomes possible that the Supreme Court chooses to take it up.

To do so, the court would need to determine that it had jurisdiction over the issue and that a candidate was bringing forward a legitimate legal challenge, such as how certain classes of ballots should be treated.

This is a high bar to overcome.

“If there are no real good theories as to why there was some major flaw in how an election was run, then I really don’t see a pathway to litigating from being behind in an election to being a winner,” said Richard L. Hasen, an election law expert at University of California, Los Angeles School of Law.

What factors matter most?

The Supreme Court could make a difference in the final outcome if it takes up a case in a state where the margin of votes is very close, elections experts say.

“The closer things are, the more you can expect a torrent of post-election litigation,” said Richard H. Pildes, an election law expert at New York University School of Law.

The margin of votes between Ms. Harris and Mr. Trump would need to be razor thin. Even a gap of about 10,000 votes, as was seen in Georgia and Arizona in 2020, would most likely not be close enough for a Supreme Court decision to make a difference in the final outcome.

The slim margin would also have to be in a state with enough electoral votes to make a difference in the final count.

Elections experts will be paying special attention to Pennsylvania, which has 19 electoral votes and which Professor Pildes said is “always a fertile ground for election litigation.”

What is the precedent for the Supreme Court deciding elections?

While the Supreme Court routinely considers election-related litigation, it has historically chosen not to get involved in ballot counting.

The major exception came in 2000, when the Supreme Court considered a dispute in Florida between then-candidates Gov. George W. Bush of Texas and Vice President Al Gore. On Dec. 12, 2000, by a 5-4 vote, the Supreme Court overturned the Florida Supreme Court and halted the recount of votes, leading Mr. Bush to win the state and ultimately the presidency.

In the end, Mr. Bush won Florida by only 537 votes.

“If we end up with a really close election like we saw in 2000, then the Supreme Court’s role could end up being dispositive,” said Professor Hasen from U.C.L.A. “Then you have a situation where the margin of error in how the election was run would exceed the margin of victory of the candidate.”

Will this election be that close?

It’s impossible to predict, but polling shows one of the closest presidential elections in the history of American politics.

Professor Hasen cautioned that close polling doesn’t always lead to close outcomes, and it’s possible that none of the battleground states will be close enough for any litigation before the Supreme Court to have an influence on the final result.

What are other possibilities?

It’s also possible that the Supreme Court takes up cases after the election that won’t affect the winner of the presidency but could affect congressional races that decide which party takes control of the Senate or the House of Representatives.

The Supreme Court could also entertain litigation that may not tip the scales in 2024 but could create lasting implications in future federal elections.

The tally — the first result of this election — was announced 12 minutes after midnight. In a hamlet where 66.67 percent of the registered voters are Republicans (the other two are independents) and where Nikki Haley swept the primary with all six votes, the general election ended in a tie: three votes for Kamala Harris, and three for Donald J. Trump.

“I didn’t see that coming,” Scott Maxwell said of the split result. His vote, for Mr. Trump, came as something of a surprise, even to him. As late as 10:30 p.m. on Monday, Mr. Maxwell said he was undecided.

Four years ago, all five votes went to Joseph R. Biden Jr. In 2016, Hillary Clinton got four votes and Mr. Trump two.

Dixville Notch was created for the sole purpose of turning the Balsams resort into a voting location. Neil Tillotson, the hotel’s owner, won free advertising for the resort, and journalists took advantage of the in-house telephone company to deliver the town’s real, albeit statistically insignificant, vote tally 12 hours before exit polls from elsewhere in the country began to trickle in.

“It was a P.R. stunt all along,” said Andrew Smith, director of the University of New Hampshire Survey Center and a co-author of “The First Primary,” a book about New Hampshire’s role in presidential elections.

In time, the event attracted hundreds of journalists, and presidential candidates including Ronald Reagan, John McCain and both George Bushes. As the stunt bloomed into a tradition, the Balsams resort atrophied. Visitor numbers and revenues declined until 2011, when the resort closed.

How do you vote in a town that was never a town, in a hotel that is no longer a hotel?

For the first few elections after 2011, voting continued inside the former hotel, but the empty, unheated buildings deteriorated quickly in New Hampshire’s harsh winters. In 2016 and 2020, voters and journalists crammed onto the porch of a small house on the property that was renovated for the event, said Coralie Stepanian, 59, an employee at the hotel.

Another crisis struck in 2019, when the number of employees living on the property dropped to four — one fewer than the minimum required by state law for an election district. By then, Les Otten, a former ski resort developer, had purchased the property, with plans to develop it into a year-round resort.

To save the midnight voting tradition, Mr. Otten renovated a boarded-up home on the property and then moved in himself. Later, he was joined by his business partner, bringing the number of residents and voters to six.

“It’s just clean, plain democracy,” Mr. Otten, 75, said this Monday of his desire to keep the event going.

The voters of Dixville Notch worked for weeks to prepare for this year’s election night. They took calls from curious journalists in New York City, Montreal and Hong Kong. They mowed the grass in front of Mr. Otten’s home, which now doubles as the voting precinct. In the kitchen, they prepared ham and cheese croissants and chocolate chip cookies as thick as wallets. In the living room, they erected risers for television journalists, chairs for photographers, and a plastic table for counting votes.

“In our backyard, the system is going to work the way it was designed,” Mr. Otten said before the votes were cast. “Six people will vote. Their votes will be counted. And once it’s done, it’s done.”

This is a point my colleague Nate Cohn has made regularly in his election race updates over the last few weeks. But it bears repeating, because a lopsided result when there is an expectation of only razor-thin margins could further fan distrust in the polls and in the electoral process itself.

“You can have a close election in the popular vote and somebody could break 315 Electoral College votes, which will not look close,” said Lee Miringoff, director of the Marist College Institute for Public Opinion. “Or you could get a popular vote that is five points” apart, he added, “which is, by today’s standards, a landslide — a word no one has used this year.”

Since 1998, election polls in presidential, House, Senate and governor’s races have diverged from the final vote tally by an average of six percentage points, according to an analysis from FiveThirtyEight. But in the 2022 midterm elections, that average error was 4.8 points, making it the most accurate polling cycle in the last quarter of a century. If polls were off this year, in either direction, by the same margin, the winning candidate would score a decisive victory.

Based on where the polling averages stood on Monday, if the polls are underestimating Ms. Harris by 4.8 points in each of the seven swing states, she would win every one of them, and a total of 319 electoral votes, compared with only 219 for Mr. Trump. If those same polls underestimate Mr. Trump by the same margin, he would win all the battleground states, for a total of 312 electoral votes. (These calculations assume Ms. Harris and Mr. Trump each win the other states they are favored in.)

Electoral votes if Harris wins all seven swing states:

319

219

Electoral votes if Trump wins all seven swing states:

226

312

The polls may not all miss in the same direction, or by that magnitude. But even a historically accurate year for the polls could mean either candidate sweeps most of the battleground states — Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

Pre-election polls have a certain amount of error baked in, partly through the polling process itself, but largely because they have to guess who is actually going to turn up to vote, in order to match the sample they have to that voting population. Other public opinion polls that focus on nonpolitical topics don’t have to deal with that challenge.

“Because the tools pollsters have to forecast turnout are fairly limited — stated intention to vote, interest in the election, perceived importance of the outcome, past turnout — they may all make the same mistakes if those tools don’t turn out to be that useful in a given election,” said Scott Keeter, a senior survey adviser at Pew Research Center.

Who shows up to vote obviously makes a big difference in terms of who wins. The biggest lead in any swing state right now is Mr. Trump’s in Arizona, where polling averages show him ahead by three points, so even a miss as small as two or three percentage points could mean a big victory for either candidate.

Part of this is because polls tend to miss in the same direction in a given election cycle (though not always), and battleground states that are demographically similar tend to vote the same way (though, again, not always). As a result, if the polls are off by even a small margin, but off in the same “direction” (either underestimating Mr. Trump or underestimating Ms. Harris), that could mean the difference between one candidate, or the other, knocking down the swing states like dominoes.

None of this is to say that we will definitely have a knockout on Election Day; there’s still the very real possibility that it will be a narrow race that comes down to a few hundred votes in a single state. But understanding that these are possibilities well within the range of normal polling error may help curb some of the whiplash when the results are finally tallied.