When Americans turn up to the ballot box next month, it could be the tightest election in more than two decades, according to a US politics expert.
The two candidates, Vice-President Kamala Harris and former president Donald Trump, have been neck and neck in most polls, with US election analysts forecasting that either candidate could win come election day.
One of those election analysts, FiveThirtyEight, explored polling as well as economic and demographic data in its analysis of the upcoming election.
In its simulations, FiveThirtyEight found Harris won 53 times out of 100, while Trump won 47 times out of 100.
Here's how this upcoming election compares to one of the closest in American history.
'When the polling is that close, it's impossible to say who has the lead'
University of Sydney associate professor David Smith, who specialises in US politics, believes there hasn't been a US election this close since 2000, based on polling.
"What we see in the national polling is that Kamala Harris has a very slight lead, but in all of the swing states, it's less than a two-point lead, and in most of them, it's less than a one-point lead," Dr Smith said.
"When the polling is that close, it's impossible to say who has the lead."
Dr Smith explained those polls were usually conducted with sample sizes of about one or two thousand people, and a 1 per cent difference between them was within the poll's margin of error.
"The fact that nearly all the polls that we see in most of the states are so close, it just makes it a lot harder to predict than any election in recent memory," he said.
"It's getting to the point where whenever I see a new poll, it doesn't really tell me anything new about who's going to win the election."
What happened in the 2000 US election?
The US election in 2000 was among the most controversial in the country's history but also one of the closest.
Republican candidate George W Bush won the electoral college but ended up losing the popular vote to the Democrat candidate by about half a million votes.
Hundreds of votes separated the candidates in Florida and vice-president Al Gore demanded a recount.
A 36-day political and legal war ensured over "how to resolve what was essentially a tie", according to the Brookings Institute.
"Bush ultimately garnered the presidency when a sharply divided and transparently political Supreme Court ended the manual recount in Florida that might have produced a different outcome," the institute said.
"It was the closest presidential election in American history, with only several hundred votes in Florida determining the winner out of more than 100 million ballots cast nationwide."
Dr Smith said there were riots around some of the vote counting stations in Florida, which later became known as the Brooks Brothers riots.
"In Florida back in 2000, the margin was fewer than 1,000 votes … so it was close enough that neither side was prepared to accept the result," he said.
But "the political atmosphere [now] has become more violent", he said, citing the attack on the US Capitol on January 6, 2021, after Trump lost the election.
"In 2020, there were a couple of results that came down to 10 or 11,000 votes," he said.
"And that was a big enough margin that none of the kinds of challenges that Republicans were mounting really stood any chance."
Why is this US election expected to be so close?
Australian National University's Wesley Widmaier referenced the increasing polarisation in US politics as one of the reasons why this US election might be close.
"Especially since the global financial crisis, it's a very polarised society … it's not just red and blue," he said.
"There's a professional educated elite which does not know how to talk to the other side of the country, and then the other side of the country looks at that professional elite and resents them.
"So that's made it very difficult to achieve consensus because you have these two world views which are in intensifying conflict with one another."
Dr Smith says US elections often come down to the swing states.
He referenced the 2020 election, when Democrat Joe Biden defeated incumbent Republican president Donald Trump 306-232 in the Electoral College and by four percentage points in the popular vote.
NPR reported that "just 44,000 votes in Georgia, Arizona and Wisconsin separated Biden and Trump from a tie in the electoral college".
Dr Smith said: "The reason why these elections appear so close is because of the electoral college system."
"It is decided within these key states, where the results can be a lot closer. If it was decided at the national level, you could still have pretty close results, but it wouldn't be anything like this close."
That's why Dr Smith said the electoral college was not a good system for deciding a presidential election.
"There's no need for geographic representation, it's the person who is representing the entire country," he said.
"It really undermines the principle that everybody's vote has the same value because it doesn't in the system of the electoral college.
"I think it's a very archaic system that is left over from a different period of American politics where the president wasn't elected directly."
