A handful of states will play a crucial role in determining the outcome of the 2024 United States presidential election.

These are the “swing states”: so called because they will help swing the overall result either towards Democrat candidate and current vice president Kamala Harris, or away from Ms Harris and towards her Republican opponent – and former president – Donald Trump.

Most states are not swing states and will not change hands at this election – but safe states alone are not enough to get either Ms Harris or Mr Trump to the White House.

Here are seven swing states to watch.

All but one were won by President Joe Biden for the Democrats in 2020, but all are being strongly contested at this election by Mr Trump.

– Pennsylvania

Donald Trump won Pennsylvania in 2016, confounding those who assumed this was a Democrat stronghold, as had been the case at every election since Bill Clinton won the state in 1992.

Joe Biden regained it narrowly for the Democrats in 2020, but polls suggest the outcome this year is on a knife’s edge.

A total of 270 electoral votes are needed to win the US presidential election, with each state allotted a fixed number of votes based roughly on the size of its population.

Whoever wins the popular vote in a state also wins all of that state’s electoral votes (with two exceptions, Maine and Nebraska, where the votes are split on a district and state-wide level).

With 19 electoral votes up for grabs in Pennsylvania, this is a key target for both candidates.

But due to the huge number of postal and early votes cast, which will need to be sorted and counted, the outcome is not likely to be clear for some time.

In 2020 the state was not called for Mr Biden until four days after election day.

– Michigan

Like Pennsylvania, this was part of the “blue wall” of states that voted Democrat at every presidential election from 1992 to 2012.

Mr Trump’s victory in 2016 was tiny – by just 0.2 percentage points – but symbolised the scale of his success in wooing working-class voters away from his opponent Hillary Clinton, particularly in a state that Barack Obama had carried by nine percentage points in 2012.

Joe Biden won back the state for the Democrats in 2020 by a gap of nearly three points over Mr Trump, but opinion polls suggest it will be much closer this time.

There are 15 electoral votes in play here.

A map showing the swing states in this year's US presidential election
Swing states in this year’s US presidential election (PA Graphics)

– Wisconsin

This is another “blue wall” state that flipped narrowly to the Republicans and Donald Trump in 2016, before switching back – by a similarly narrow margin – to the Democrats and Joe Biden in 2020.

As is the case with Pennsylvania and Michigan, the result in Wisconsin might not be clear for several days, thanks to the volume of postal and early votes that will need to be sorted and counted.

Ten electoral votes are available here.

– Arizona

In 2020 Joe Biden was the first Democrat to win Arizona since Bill Clinton in 1996.

This represented a major turnaround from 2016, when Donald Trump beat Hillary Clinton in the state by more than three percentage points.

Mr Biden’s victory was very slim and this year the state once again looks like being a nail-biter, with polls suggesting the outcome is too close to call.

Arizona is worth 11 electoral votes.

– Nevada

Nevada has been won by the Democrats at every presidential election since 1992, but often by close margins – and this year, it could be closer than ever, with polls suggesting the race is effectively tied.

Joe Biden won the state in 2020 by just over two percentage points.

Nevada does not offer a huge number of electoral votes – six – but in a close contest, a victory here could be decisive in helping either Ms Harris or Mr Trump get to the White House.

A bar chart showing how the swing states in the US presidential election voted in 2020
US presidential election: how swing states voted in 2020 (PA Graphics)

– North Carolina

This is the one swing state on the list that was won by Donald Trump in 2020, though the result was close, with Mr Trump picking up 49.9% of the vote and Joe Biden 48.6%.

North Carolina has not been won by a Democrat since Barack Obama took the state in 2008 – the first Democrat to do so since Jimmy Carter in 1976.

Once again, opinion polls this year point to a close contest, with a valuable 16 electoral votes up for grabs.

North Carolina is the swing state most likely to report prompt results on election night, and as such will give an early indication of how both Ms Harris and Mr Trump are faring.

– Georgia

Joe Biden in 2020 was the first Democrat to win Georgia since Bill Clinton in 1992.

Barack Obama never came near to a victory, and Donald Trump won it by more than five percentage points in 2016.

The result in 2020 was so close that a recount was conducted, with the outcome not confirmed until more than two weeks after election day.

Just 0.3 percentage points separated Mr Biden from Mr Trump.

Like North Carolina, the state gives its winner 16 electoral votes and opinion polls here, as elsewhere, point to another incredibly close contest.

But polls are only snapshots, not predictions, and ultimately the outcome in places like Georgia may come down to which side manages to get more of its supporters to turn out and vote.