Explained | Why we may not know the winner of the US presidential election on November 5

The US presidential election will take place on Nov. 5, but the winner of the razor-thin race between Democrat Kamala Harris and Republican Donald Trump may not be known for days after the polls close.

As ballots are counted, one candidate may appear to be leading based on early returns, only for a rival to close the gap as more votes are tallied.

In 2020, some states experienced a "red mirage," in which Trump appeared to be leading on election night, before a "blue shift" saw Democrat Joe Biden overtake him, a phenomenon Trump used to amplify his false claims that the election was stolen.

Nothing untoward had occurred. Democrats tend to live in more populous urban areas, where counting votes takes longer. Democrats also have embraced mail voting more readily than Republicans after Trump's false claims that mail ballots are untrustworthy, and those ballots take longer to count than Election Day votes. Trump has both encouraged and criticized early and mail-in voting in 2024.

Democrats are outpacing Republicans in mail ballots once again this year, according to an early vote tracker maintained by the University of Florida's Election Lab, though Republicans have narrowed the gap.

There are seven battleground states likely to decide the election, each with its own rules for handling and counting ballots. Here's what to expect on Election Day and beyond:

Arizona

Voting by mail is extremely popular in Arizona; nearly 90 per cent of voters cast their ballots early, most by mail, in 2020. Election officials in Arizona can begin processing and tabulating mail ballots upon receipt, but results cannot be released until one hour after polls close.

Any mail ballots dropped off on Election Day itself cannot be processed until the polls have closed. That is often a sizable number – in 2022, those "late early" votes comprised one-fifth of all ballots in Maricopa County, the state's largest – and can take days to count.

The initial results on election night should be mostly early votes, which could favor Harris, before the numbers shift toward Trump as Election Day votes are tallied.

Georgia

Early in-person voting is popular in Georgia, where officials expect 65 per cent to 70 per cent of ballots to be cast at early poll locations. Absentee or mail ballots, which may comprise around 5 per cent of the vote, can be processed - which includes steps such as verifying signatures - starting two weeks before the election, though workers must wait until Election Day to begin counting them.

All early votes – in-person and mail – must be counted and reported by 8 pm ET (0000 GMT) on election night, according to state law, which could create a "blue mirage" in Harris' favor at first. Officials are aiming to have all votes, including those from Election Day, tallied by midnight.

Ballots from overseas and military voters will be accepted up to three days after the election if postmarked by Nov. 5. There were more than 21,000 such ballots requested, so an extremely close election might not be resolved until those votes are tabulated.

Michigan

Since the 2020 election, Michigan has instituted early in-person voting for the first time and begun permitting jurisdictions with more than 5,000 people to begin processing and tabulating mail ballots eight days before Election Day. Smaller jurisdictions can do so the day before Nov. 5.

Officials hope those changes will allow the state to report results more quickly than in 2020, when mail ballots could not be processed in advance. That created a "red mirage" on election night, when the state's early counts of Election Day votes favored Trump. Biden eventually surpassed Trump on the strength of mail ballots, which took longer to tally. Trump falsely claimed he was the victim of fraud.

Nevada

Nevada's slow vote counting in 2020 – news outlets did not 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 notably, counties were permitted to begin processing and counting mail ballots on Oct. 21. In addition, workers can start tabulating early in-person votes at 8 am PT (1500 GMT) on Election Day, rather than waiting until polls close.

But Nevada still might not get called right away. Mail voting has grown popular in the state, and it is the only battleground that accepts late-arriving mail ballots. That could also create a "blue shift" as more votes are counted.

Any ballot postmarked by Nov. 5 will still be counted if it arrives within four days. Those late ballots historically favor Democrats, so a shift toward Harris could occur as votes are counted after Election Day.

North Carolina

Election officials start processing and scanning mail ballots ahead of Election Day. After polls close, the first reported results will likely be mostly mail ballots as well as early in-person votes. Election Day votes will be counted and reported throughout the evening, with full results expected by midnight.

Harris may appear to lead early thanks to mail ballots, while Trump could close the gap as Election Day votes are counted.

If the election is as close as polls suggest, the outcome in North Carolina may remain unclear for a week or more. Absentee ballots that arrive on Nov. 5, as well as ballots from overseas and military voters, are tallied during the 10-day canvass period that follows Election Day. In 2020, media outlets did not call North Carolina for Trump until Nov. 13, 10 days after the election.

Pennsylvania

Perhaps the most important battleground, Pennsylvania did not have a clear winner in 2020 for four days after Election Day, as officials sifted through a huge backlog of mail ballots. The state is among only a handful that do not permit election workers to process or tabulate mail ballots until 7 am ET on Election Day, which means it will likely again take days before the outcome is known.

With more Democrats than Republicans voting by mail, the early results – based on in-person Election Day votes – will probably show Trump ahead, but his lead will likely shrink as more mail ballots are counted.

That pattern in 2020 prompted Trump to falsely claim fraud. This year, a new law requires most counties to announce at midnight on election night how many mail ballots remain to be counted in an effort to forestall conspiracy theories.

Wisconsin

Like Pennsylvania, Wisconsin is among the few states that do not allow election officials to process or count mail ballots until the morning of the election, which means there can be a delay in reporting the results of those early votes.

In addition, many of the state's largest cities transport mail ballots to a centralized location for processing and tabulating. That can lead to significant batches of votes getting reported all at once in the early morning after polls close.

In 2020, Trump and his allies falsely claimed fraud after Milwaukee, the state's largest city, reported nearly 170,000 absentee ballots around 3:30 am CT (0830 GMT), giving Biden a huge spike that moved him into the lead for the first time. But that increase was expected due to the way the city processes those ballots and the fact that Democrats were more likely to vote by mail. A similar pattern is probable in 2024.