Under the country's Electoral College system, each state has a set number of "electors." The number of votes each state has is determined by how many members of Congress represent that state. For example, a state with three House members and two senators has five Electoral College votes. In almost every state, all of the state's electors vote for the candidate who wins the most votes in that state, which is known as "winner-take-all."

But that's not the case in Nebraska, where three of the state's five electors vote according to the result of their congressional district.

In 2020, Joe Biden lost to then-President Donald Trump by 19 points in Nebraska. But Biden defeated Trump by 6.5 points in the 2nd district, which encompasses the state's largest city, Omaha. As a result, four of the state's five electoral votes went to Trump, while one went to Biden.

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Several polls have shown Harris leading in the 2nd district this year, and Republicans have been looking to avoid a repeat of 2020. On Wednesday, Sen. Lindsey Graham of South Carolina skipped Senate votes to travel to Lincoln, where he lobbied state lawmakers to switch the state to winner-take-all in the final weeks before the election.

"I was glad to go out and talk about the world as I see it," Graham told CBS News on Thursday. "I hope the people in Nebraska will understand this may come down to a single electoral vote. And I just don't believe a Harris presidency is good for Nebraska."

The Electoral College math

Under the Electoral College, a candidate needs to win 270 votes — a majority of the 538 total electors — in order to win.

Seven states are widely considered to be the key swing states. Three of them are the traditional "blue wall" states that have historically voted Democratic, but that Trump narrowly carried in 2016: Pennsylvania, Michigan, and Wisconsin.

The other four are the so-called "Sun Belt" states of Nevada, Arizona, Georgia, and North Carolina. Biden won Nevada, Arizona, and Georgia in 2020, but Trump has generally led in polling of those states this year.

There's also Maine, a Democratic-controlled state that apportions its electors the same way as Nebraska. Trump comfortably won Maine's 2nd district in both 2016 and 2020, and though some polling has shown Harris leading Trump there, he's generally considered more likely to win that district, giving him one of the state's four votes.

Maine earlier this year threatened to switch to a winner-take-all system for Electoral College votes if Nebraska did the same. But unlike in Nebraska, it's probably too late to do that before the election — it would require two-thirds support from the state legislature, which Democrats don't have.

How one vote could change the election

As Graham noted, just one Electoral College vote is at stake in Nebraska. But that single vote could be the difference between a Harris win and a tie vote.

Under the current rules, if Harris were to lose all of the Sun Belt states and Maine's 2nd district, but won all of the blue wall states plus Nebraska's 2nd district, she would win exactly 270 votes — enough to become president.

But without that one vote from Nebraska, Harris would need to win at least one of the Sun Belt states or Maine's 2nd district.

Otherwise, it's a 269-269 tie — a result that would throw the election toward the House of Representatives, where a candidate must win the support of at least 26 state delegations to win. Under such a scenario, Trump is strongly favored — Republicans are likely to control most state delegations, even if they don't have a majority.

That's not to say it's impossible for Harris to win outside the blue wall. The vice president has generally led in Nevada polling, with Trump only narrowly ahead in the other three states. Harris campaign chair Jen O'Malley Dillon wrote in a memo at the beginning of this month that though they consider themselves to be the "clear underdogs," they believe Harris has "multiple pathways to 270 electoral votes."

But changing Nebraska's Electoral College apportionment would make an already tough election even tougher for Harris, denying her the ability to merely rely on the "blue wall" to win.

It remains unclear if Nebraska lawmakers will press ahead with the switch to winner-take-all. Republican Gov. Jim Pillen supports it, but there weren't enough votes in the state's unicameral legislature to make the change earlier this year.