It is hard to predict when we’ll know the winner of the 2024 US presidential election, as voters head to the polls across America with a choice between Republican ex-president Donald Trump and Democratic vice-president Kamala Harris. If the result is clear cut, it may well come later tonight; if, as expected, the election proves to be a knife-edge race, there could be significant delays.

Follow AS’s live coverage of Election Day and the count in swing states:

How long could it take to reach a US election result?

In recent years, the speed with which the US election result has become known has varied greatly. In 2012, for example, Democratic incumbent Barack Obama beat Republican challenger Mitt Romney so comprehensively (taking 337 electoral votes to Romney’s 206) that media outlets were already calling Obama as the winner before midnight on Election Day. Four years later, when Republican Donald Trump claimed a surprise victory over Hillary Clinton - winning by 304 electoral votes to 227 - the Democratic nominee had called Trump to concede by 3:00 a.m. ET on the morning after the election.

In 2020, however, things took a while longer. Indeed, it wasn’t until November 7, four days after the election, that media outlets declared Democrat Joe Biden as the winner in Pennsylvania and, by extension, the victor in his presidential contest with Trump, then the incumbent commander-in-chief.

What can cause delays to the election outcome?

A swing state that is again poised to be a key battleground this year, Pennsylvania was chiefly delayed in declaring a result in 2020 by the large number of mail-in ballots its election officials had to process. Per CBS, an unprecedented 375,000 Pennsylvanians opted not to vote in person, in an election that came amid the global covid-19 pandemic.

With the pandemic now over, fewer mail-in ballots were expected across the US this year. However, concern over the potential for delays in the handling of such votes has remained, as can be seen in recent bids to pass laws allowing clerks to begin processing absentee ballots ahead of Election Day in states where this is not permitted. Such legislation has, unsuccessfully, been tabled in places such as Pennsylvania and Wisconsin, which is another key ‘toss-up’ state.

In states where there are tight races, recounts may also mean we have to be patient in our wait for the Oval Office’s new occupant to be declared. According to the National Conference of State Legislatures, 24 states and the District of Columbia all require automatic recounts if there is only a very close margin of victory. The threshold is normally 0.5% or below, but is sometimes 1% or under.

In this year’s contest between Trump and Harris, there are a number of states in which the two presidential contenders are likely to be neck and neck. As of Monday, indeed, the data analysis specialists RealClearPolling estimated a difference of less than one percentage point in no fewer than five swing states.

And in such a tight race for the White House, the expectation is that there will be post-Election Day legal challenges - particularly from the Republicans - with the potential for the result to be held up. After all, there has already been a wealth of pre-election litigation: citing the tracking website Democracy Docket, Reuters notes that there have been just under 200 such lawsuits in 2024.

Reuters adds that Trump and his allies lodged over 60 lawsuits challenging the results of the election four years ago. While these cases were unsuccessful, there is precedent for the outcome of a close election being delayed for weeks amid court battles. In 2000, George W Bush was not declared the winner until the US Supreme Court ruled to end a recount in Florida on December 12 - over a month after Election Day. The Supreme Court’s ruling confirmed Bush as the winner of the state, with a mere 537 votes more than his Democratic opponent, Al Gore, and took the Republican over the threshold of electoral votes needed to clinch the presidency.

“Huge risk for spread of mis- and disinformation”

In 2020, the Trump team grounded its many legal challenges on baseless claims of electoral fraud - and if this year again sees certain states take longer to tally every vote, it feels inevitable that the ex-president and his supporters will once more seize on such delays as evidence of corruption.

“We could have a situation where we just have a couple of bottlenecks,” Rachel Orey, senior associate director for the Bipartisan Policy Center’s Elections Project, told an interview with NBC News in February. “That creates huge risk for the spread of mis- and disinformation when you have a couple of states in the spotlight, and you have candidates saying - well, we have results from all these other states. Why are you taking so long? There must be something wrong […].

"Ultimately, the responsibility comes down to the candidates. But that’s easier said than done.”