Legal challenges are also possible. More than 100 pre-election lawsuits have already been filed by Republicans, including challenges to voter eligibility and voter roll management.
Other scenarios that could cause delays include any election-related disorder, particularly at polling locations.
On the other hand, vote-counting has sped up in some areas, including the crucial state of Michigan, and far fewer votes will be cast by mail than in the last election, which was during the Covid pandemic.
In the 2020 election, US TV networks did not declare Joe Biden the winner until four days after election day, when the result in Pennsylvania became clearer.
In other recent elections, voters have had a much shorter wait.
In 2016, Trump was declared the winner shortly before 03:00 EST (08:00 GMT) a few hours after polls closed.
In 2012, when Barack Obama secured a second term, his victory was projected before midnight the same evening of election day.
However, the 2000 election between George W Bush and Al Gore was a notable exception. The race was not decided for five weeks, when the US Supreme Court voted to end Florida’s recount. That kept Bush in place as winner and handed him the White House.
What are the swing states to watch and when might they declare?
The race is expected to come down to results from seven swing states, which experts believe Harris and Trump both have a realistic chance of winning.
Turnout has been high in early voting, both in-person and by mail, with records broken in Georgia.
Georgia – Polls close in the Peach State at 19:00 EST (00:00 GMT). Georgia’s top election official estimates that about 75% of votes will be counted within the first two hours.
North Carolina – Polls close 30 minutes after Georgia. North Carolina’s results are expected to be announced before the end of the night.
Pennsylvania – Voting ends at 20:00 EST (01:00 GMT) but experts agree it may take at least 24 hours before enough votes are counted for a winner to emerge.
Michigan – Voting concludes at 21:00 EST (02:00 GMT). A result is not expected until the end of Wednesday.
Wisconsin – Results should come in shortly after polls close at 21:00 EST for smaller counties but experts predict the state won’t have a result until at least Wednesday.
Arizona – Initial results could come as early as 22:00 EST (03:00 GMT) but the state’s largest county says not to expect results until early Wednesday morning. Postal ballots dropped off on election day could take up to 13 days to count.
Nevada – Votes here could also take days to count. The state allows mail-in ballots as long as they were sent on election day and arrive no later than 9 November.
