WASHINGTON (Reuters) – Early vote returns in U.S. battleground states may not be a good indicator of whether Democratic candidate Kamala Harris or Republican challenger Donald Trump will win because of vote counting rules and quirks in some key states, experts say.

In the 2020 election, some states showed a “red mirage” in which Trump was ahead on election night before a “blue swing” in which Democrat Joe Biden overtook him as mail-in ballots favored by more Democratic voters were counted.

Experts correctly predicted this would happen, but Trump still used the shift to bolster his baseless claims that the election was stolen.

It may happen again this week, especially in Pennsylvania, Michigan and Wisconsin.

The opposite (a “blue mirage” that apparently shows a strong outcome for Harris in terms of a “red shift” that will only reverse the trend) could also happen this week, especially in North Carolina and Georgia.

Trump’s campaign has said he plans to declare victory once his team believes he has passed the 270 Electoral College votes needed to win the election. This may still be premature, depending on the number of votes yet to be counted and where they come from.

How it all plays out will depend on how and when large numbers of mail-in or early votes are processed and counted in different states.

Another factor is that Democrats tend to live in more populous urban areas where vote counting takes longer.

There are seven battleground states that are likely to decide the election, and each has its own rules for processing and counting votes. What to expect on Election Day and beyond:

PENNSYLVANIA

Pennsylvania remained without a clear winner in 2020 for four days after Election Day as officials sifted through a massive pile of mail-in ballots.

The state is one of the few that does not allow election workers to process or tabulate mail ballots until 7 a.m. on Election Day; This means that it may still take days before the result is known.

With more Democrats than Republicans voting by mail, initial results based on in-person Election Day votes will likely show Trump ahead, though his lead is likely to shrink as more mail votes are counted.

This pattern in 2020 led to Trump claiming fraud. This year, a new law requires most counties to announce how many mail-in ballots are left to be counted at midnight on election night in an effort to prevent conspiracy theories.

Wisconsin

Like Pennsylvania, Wisconsin does not allow election officials to process or count mail ballots until election morning; This means there may be a delay in reporting the results of early voting.

In addition, many of the state’s largest cities have a central location for processing and tabulating mail-in ballots. This could lead to a significant amount of votes being reported at once early Wednesday morning.

In 2020, Trump and his allies claimed fraud after the state’s largest city, Milwaukee, reported nearly 170,000 absentee ballots around 3:30 a.m. CT (0830 GMT), giving Biden a big boost that propelled him into the lead for the first time.

This increase was expected because of the way the city processes ballots and the fact that Democrats are more likely to vote by mail. A similar situation is likely to occur in 2024.

GEORGIA

Early in-person voting is popular in Georgia, with officials expecting 65 percent to 70 percent of votes to be cast at early polling stations.

Absentee ballots, or mail-in ballots, which can account for about 5% of the vote, can be processed two weeks before the election, which involves steps such as verifying signatures, but workers must wait until Election Day to begin counting.

By state law, all early votes (in person and by mail) must be counted and reported by 8pm ET (0000 GMT) on election night. Officials aim to have all votes counted by midnight, including those on Election Day.

Ballots from overseas and military voters will be accepted within three days of the election if postmarked by Nov. 5.

More than 21,000 such ballots were requested; thus, an extremely close election may not be possible to resolve until these votes are tabulated.

ARIZONA

Voting by mail is extremely popular in Arizona; In 2020, nearly 90% of voters cast their ballots early, mostly by mail. Election officials in Arizona can begin processing and tabulating mail ballots once they receive them, but the results may not be released until an hour after polls close.

Ballots sent by mail on Election Day cannot be processed until polls close. This is usually a pretty large figure; In 2022, these “late early” votes accounted for one-fifth of all votes in Maricopa County, the state’s largest ballot box, and could take days to count.

The first results on election night will be mostly early votes, which could favor Harris before the numbers shift toward Trump as Election Day votes are counted. They could return to Harris in the coming days as late-arriving mail votes are tabulated.

MICHIGAN

Michigan has introduced early in-person voting for the first time since the 2020 election, allowing jurisdictions with more than 5,000 people to begin processing and tabulating mail ballots eight days before Election Day.

Smaller jurisdictions can do this the day before November 5th.

Officials hope these changes will allow the state to report results more quickly than in 2020, when mail-in ballots could not be processed in advance.

This created a “red mirage” on election night, with early counts of the state’s Election Day votes favoring Trump. Biden eventually surpassed Trump on the strength of mail-in ballots, which take longer to count. Trump claimed he was the victim of fraud.

NORTH CAROLINA

Election officials begin processing and scanning mail-in ballots ahead of Election Day. The first results reported after polls close will likely be mostly mail-in voting as well as early in-person voting.

Election Day votes will be counted and reported throughout the evening, with full results expected by midnight.

While Harris may appear to be ahead early thanks to mail-in voting, Trump could close the gap as Election Day votes are counted.

If the election is as close as polls indicate, the outcome in North Carolina could remain uncertain for a week or more. The counting of ballots received on November 5, as well as votes from abroad and military voters, is carried out during the 10-day voting period following Election Day.

In 2020, media outlets did not call North Carolina for Trump until November 13, 10 days after the election.

NEVADA

Nevada’s slow vote count in 2020 — news outlets didn’t call the state for Biden until five days after Election Day — launched countless memes, but officials say changes since then should speed up the process.

Most importantly, counties were allowed to begin processing and counting mail ballots on October 21.

Workers can also begin voting in person as early as 8 a.m. PT (1500 GMT) on Election Day, rather than waiting for polls to close.

But Nevada still might not get a quick call. Voting by mail has become popular in the state, and it is the only battleground state that accepts late-arriving mail ballots.

Ballots postmarked by November 5 will still be counted if they arrive within four days. Since these late votes have historically favored Democrats, a shift toward Harris could occur as votes are counted after Election Day.

(Reporting by Washington Newsroom; Editing by Lincoln Feast.)