WaPo:

Shortly after Election Day came and went in 2020 with no presidential decision, then-president Donald Trump declared himself the winner, even though millions of votes had yet to be counted. A little more than three days later, the tally would show that he had lost.

As Trump saw his early leads dwindle in some battleground states, he publicly called for officials to stop counting votes and falsely claimed that the remaining uncounted ballots were fraudulent.

Despite Trump’s rhetoric, it’s not unusual for a candidate’s lead in a tight race to disappear when more votes are counted. That’s called a “shift,” and it’s not a sign of wrongdoing or voter fraud, which is extremely rare, but the reality that elections officials count some batches of ballots earlier than others.

In 2020, the earliest votes counted in some key states leaned Republican, causing a “red mirage”; the later ones leaned Democratic, leading to a “blue shift.”

Blue shifts were especially noticeable in swing states during the 2020 presidential election, when Joe Biden’s margin of victory came down to thousands of votes among the millions cast.

Voting patterns and rules have changed since 2020, but if any races shift this year between bedtime and breakfast — or between Tuesday and later in the week — these are the most likely reasons….

Fewer people have cast early ballots this year so far compared with 2020, and more of the early voters appear to be Republican. The Trump campaign has been encouraging early voting, although Trump himself still sometimes disparages it.

Edward B. Foley, the Ohio State election law professor who coined the term “blue shift” in 2013, said in an email that he expects a smaller blue shift in Pennsylvania this year.

Michigan will allow officials to begin processing mail-in votes eight days before Election Day in larger jurisdictions, which could speed the count and reduce or eliminate a blue shift there. Michigan Secretary of State Jocelyn Benson (D) has said she expects unofficial results in her state by the end of Wednesday at the latest.

However, North Carolina, which historically counts quickly and shifted red on election night in 2020, is a wild card this year as many residents deal with the aftermath of Hurricane Helene. And when an election is tight, time-consuming recounts are more likely, as in Georgia in 2020.