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The three swing states of Pennsylvania, Michigan and Wisconsin are part of the Rust Belt – former heartland of the US manufacturing industry.

That industrial heritage means many voters in these states have traditionally been union members with a long history of voting for the Democrats – and the Democrats have taken all three states in seven of the last eight presidential elections.

When they did flip for Donald Trump in 2016, they all flipped together.

So history would suggest that one candidate will likely take all three states – and if that candidate is Kamala Harris, then she wins the election.

If Donald Trump swept up these three states again, he would still need to pick up at least one state elsewhere.

For Harris the key to these states – as with many states across the country – is to pile up votes in and around the cities by ensuring that black, minority and college-educated voters turn out on election day along with middle-class voters in the suburbs – particularly white women.

Trump's strength in these states is the opposite. He won the 2016 election by amassing huge winning margins in the rural areas of these states – appealing to those voters outside the cities.

Trump is hoping he can find even more votes in rural areas this year and his campaign is focusing on persuading younger men who may have never voted before to get to the polls. If they turn out in large numbers, it could further bolster his white working-class vote in the Rust Belt.

HARRIS KEY STATE: Pennsylvania

Pennsylvania is the biggest prize of the 2024 swing states with 19 electoral votes.

If Kamala Harris can win Pennsylvania plus one more from this Rust Belt group, she leaves Donald Trump needing close to a clean sweep of all the other swing states.

With Pennsylvania, Harris could pass 270 by winning just three swing states overall. Nevada doesn't have enough electoral college votes for this scenario, but Pennsylvania and any other two would be enough.

A Pennsylvania election

This is Pennsylvania's presidential election. The other states just live in it. According to some projections, the candidate who wins Pennsylvania has upwards of a 90% chance of winning the White House. It's that important. Pennsylvania, in a way, is a microcosm of the United States. It has big cities dominated by Democrats and Republican-friendly rural areas. Its demographic makeup is similar to the country overall. It has new technology companies and old industries; energy extraction and agriculture. A candidate can't be a one-trick pony and win in Pennsylvania. It is the place that both campaigns have been investing the most time and resource. And polls almost universally show the state sitting on a knife's edge.

Anthony Zurcher

Â鶹¹ÙÍøÊ×Ò³Èë¿Ú North America correspondent

The states in the southern US are often referred to as the Sun Belt for the obvious reason that they have a hotter climate than those in the north.

There are four swing states in the south at this election: Arizona, Georgia, Nevada and North Carolina. As with the Rust Belt, Harris would become president if she won all four.

However, it feels an unlikely path to victory for Harris as the Democrats haven’t won all four of these states since Harry Truman was elected in 1948.

If Trump swept all the swing states in the Sun Belt, he would still need to pick up one more state.

But, unlike the Democrats, Republicans do have a history of sweeping these states. Since the Civil Rights Act upended the political map in the 1960s: Presidents Nixon, Reagan, Bush and Bush Jr each won all four of these states at one or more elections.

There are two key groups that could help decide the results in the Sun Belt: black and Latino voters.

In both Georgia and North Carolina a large percentage of eligible voters are black. Harris will hope she can match Joe Biden’s performance with those voters; he won 92% of all black votes across the US in 2020, but some recent polls have suggested Harris is struggling to hit similar numbers.

Arizona and Nevada have fast-growing Latino populations and for a long time, Democrats assumed an increase in Latino voters in any state would automatically increase their chances of winning. But Donald Trump surprised many by making in-roads with Latino voters at the last two elections, with his perceived strength on the economy being particularly attractive.

However, Republicans are concerned that widely-reported comments about Puerto Ricans and other Latinos by speakers at Trump's recent Madison Square Garden rally, which leant heavily on racist stereotypes, could undo some of those gains.

TRUMP KEY STATE: North Carolina

Donald Trump held North Carolina in the last two elections and it wasn’t considered a swing state until Kamala Harris took over the Democratic ticket.

If Trump failed to hold North Carolina, his route to the White House becomes much harder as he would need to win at least two of the Rust Belt states and two more in the Sun Belt.

The state has only voted for the Democratic candidate once since 1980 – for President Obama in 2008.

But there is a chance Harris could take it if she can inspire black voters to turn out, as they did for Obama, and she picks up votes from the recent influx of college-educated voters.

Chasing persuadables

Campaign aides for Harris believe that around 10% of voters in all seven battleground states are still persuadable, and they haven't given up on changing hearts and minds in an election where the slightest shift in a swing state could have major electoral implications. In North Carolina, a state Democrats still believe they can flip despite an uphill battle, Harris has focused on Republican college-educated, suburban women who are turned off by the former president's rhetoric. Meanwhile, the Trump campaign has grown more confident about Georgia and North Carolina, and the former president has also set his sights on becoming the first Republican presidential nominee to win Nevada since George W Bush won it in 2004.

Courtney Subramanian

Assistant editor, Â鶹¹ÙÍøÊ×Ò³Èë¿Ú News, Washington

At the last two elections the winning candidate - Donald Trump in 2016 and Joe Biden in 2020 - won six of the seven 2024 swing states.

Trump missed out on Nevada by only 27,000 votes, and Biden lost in North Carolina by around 75,000 - having improved on Hilary Clinton’s total there in 2016 by over 100,000 votes.

A small difference in turnout could have seen Trump or Biden sweep all these states, and at this election it's possible either candidate could do the same.

Neither of them has a decisive lead in any of the seven swing states at the moment but the margin of error in each poll means one of them could be further ahead than it appears.

If polls have underestimated support for Trump like they did in 2016 and 2020 then he could well end up winning all seven swing states and even picking up another state or two elsewhere in the country.

But equally, if pollsters' assumptions about turnout are wrong and enthusiasm within the Democratic Party helps bring record numbers of Kamala Harris supporters out to vote in key states, then she could end up with a winning map that looks like Biden's in 2020.

Polls and predictions

The polls suggest this could be one of the closest US elections in history - but should we be surprised if it isn't? Not really. In fact, we should expect the polls to be slightly out because they’re designed to broadly explain how the public feels about a candidate, not predict the result in a swing state by less than a percentage point. The average gap between Harris and Trump across all seven swing states at the moment is just over one percentage point. The average state-level polling error in the last six presidential elections was just over four points. Keep that in mind when you're thinking about possible outcomes in this election.

Mike Hills

Senior journalist, Visual Journalism team

There are combinations of states that could leave the final result with Harris and Trump tied on 269 votes each.

The most frequently discussed of these is Kamala Harris failing to pick up the single vote she is expected to win in Nebraska.

Nebraska is one of only two states which assign some of their electoral college votes to the winners in individual districts, rather than giving all of them to the candidate with the most votes in the state as a whole.

Without that single vote from Nebraska’s second district, the Democrats most likely path to victory - winning all three Rust Belt states - would result in a tie instead.

The president is then decided by a vote in Congress with each state getting a single vote - unlike the electoral college where states with more citizens get more votes.

This would almost certainly result in Donald Trump being elected president, as the Republicans tend to win more states at each election, despite the Democrats receiving more votes in total across the country at seven of the last eight presidential elections.