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It’s been called the “red mirage” or the “blue shift.”

It’s the recent phenomenon by which an apparent Republican lead early after polls close on election night is erased by the counting of mail-in ballots later in the evening or in the days after Election Day.

Former President Donald Trump has pointed to the red mirage to back up his baseless allegations of election fraud, when in fact it has been a function of the rise of mail-in voting and the often-peculiar rules about when those ballots can be counted.

What happened in 2020?

When most Americans went to bed on election night in 2020, on November 3, the end result was far from clear.

The contest between then-President Trump and now-President Joe Biden was still too early to call in the key states of Arizona, North Carolina, Nevada, Wisconsin, Michigan, Maine, Georgia and Pennsylvania.

The following days would include dramatic hours spent watching the counting of votes in these states. The vote count didn’t reflect a Biden lead in Georgia until early in the morning of November 6, when, as CNN’s Phil Mattingly showed viewers on the Magic Wall, small batches of votes were being counted and affecting the very close election.

CNN was able to project that Biden would win the election four days after Election Day, on November 7, but counting would continue. An analysis by researchers at the Massachusetts Institute of Technology found that counties won by Biden counted slower, on average, than counties won by Trump.

There was also a blue shift in 2016, but it was not decisive. As CNN’s Marshall Cohen wrote, when Hillary Clinton offered her concession and admitted her loss to Americans the day after Election Day, she was still behind in the popular vote. While the trickle-in of ballots was not enough to get her the White House, it was more than enough to give her an edge of millions of ballots in the popular vote.

In 2020, the slowest-counting states were places like California, which are heavily Democratic and where every registered voter is sent a mail-in ballot. That means a blue shift in the popular vote could still occur even if it does not delay figuring out the presidential election results.

It’s important to remember that while news networks like CNN might project a winner when it is clear who will win, the races are not officially certified until later. Certification deadlines vary from state to state, but they all have until December 11, 2024, to complete recounts, if needed, and settle disputes around presidential results.

How long will it take this year?

There is some indication that things could go quicker in some key states.

In Georgia, a new election law means more people will vote early in person rather than by mail, which could speed up the process of counting mail-in ballots. North Carolina no longer accepts mail-in ballots if they arrive after Election Day.

The Brennan Center for Justice, which tracks election-related laws, has a rundown of potential choke points.

Georgia Secretary of State Brad Raffensperger, a Republican, said recently on CBS’ “Face the Nation” that he expects the vast majority of ballots to be processed and counted in the hours after polls close.

“All early votes and all early accepted ballots, they all will have to have their results reported by 8 p.m. That’s 70, maybe even 75% of all the vote totals will be reported no later than 8 p.m. on election night,” he said. But if the election in Georgia is as close in 2024 as it was in 2020, the outcome may still not be clear.

Raffensperger was asked if it could take a week to determine the winner in Georgia.

“Absolutely not,” he said. “What we will be waiting for is the overseas ballots that come in no later than Friday, and so those will then be the final numbers. And we’ll just see if that makes the difference in the total vote totals.”

Other states, like the key battlegrounds of Pennsylvania and Wisconsin, have not made it possible for election officials to process any ballots before Election Day. But the count could be faster in those places as well, with more experience and fewer ballots to handle when compared with the pandemic-era 2020 election. The Center for Election Innovation & Research has a rundown of the various rules and timelines for processing mail-in ballots.

In Michigan, another key state where processing of ballots can occur before Election Day, Secretary of State Jocelyn Benson was also asked by CBS about how long it will take to count ballots. Benson, a Democrat, said she hopes to know the outcome by the end of Wednesday, November 6, a day after polls close.

“We will always prioritize accuracy and security over efficiency,” she said.

Close elections can take time to figure out

In the event of tight margins, it could still take days.

The top elections official in Pennsylvania is Republican Al Schmidt, who told NPR recently that people need to trust the system in the period after polls close.

“In 2020 … we saw that window of time between the polls being closed at 8 p.m. on election night and the race being called as a period of vulnerability where people were seeking to undermine confidence in the results,” he said, noting that in the end, after all the votes were counted, the Pennsylvania results were not that close.

The closest elections can take time, as anyone who is old enough to remember 2000 can attest. A close election in Florida was paired with a flawed punch-ballot system that left some ballots with “hanging chads” up for debate.

The outcome of the election that year dragged on for more than a month – 36 days until the US Supreme Court halted a partial Florida recount and essentially handed the election to George W. Bush. Then-Vice President Al Gore conceded on December 13 and then became one of just three vice presidents to oversee the counting of electoral votes in their own defeat. Harris could be the fourth. Or the first since George H.W. Bush to oversee their own victory.

In the same way that no one knows who will win the 2024 election, it’s also not entirely clear how long determining the winner this year will take.