An election ‘too close to call’ is a cliché as old as democracy. But this year’s US election really is.

Despite the Democrats’ remarkable turnaround in the polls after Kamala Harris replaced Joe Biden as nominee in August, the US Electoral College system – and Donald Trump’s loyal fanbase – has by no means guaranteed her victory.

This is because, although it’s the presidential and vice presidential nominees on the ballot paper, technically, Americans vote for electors who formally back their choice of candidate.

States have different numbers of electors depending on their size – with 538 in total. Winning the presidency requires getting at least 270 (more than half).

Historically, most states almost never change hands. Taking these so-called ‘safe’ states together, Ms Harris is just ahead of Mr Trump with 225 Electoral College votes to his 219.

Beyond that, the two parties focus almost their entire campaigns on just seven ‘swing’ states that determine the final outcome. They are Pennsylvania, Michigan, Wisconsin, North Carolina, Georgia, Arizona, and Nevada.

Pennsylvania alone often holds the key to the White House. This is because it has the most Electoral College votes of the swing states and whoever wins it is also likely to win neighbouring battlegrounds Michigan and Wisconsin – as they have similar demographics.

Here we look at the different ways both candidates could get to that magic number of 270 Electoral College votes.

In 2020, Joe Biden took all the swing states apart from North Carolina. This year, the polls there are neck-and-neck.

Although a 2020 re-run scenario would not require Ms Harris to take North Carolina, she would have to win Georgia.

Some polling has suggested the Democrats have lost support among black and Hispanic voters since the last election, which makes holding both Georgia and Arizona more difficult – particularly with Mr Trump’s support bases there.

Ms Harris is also leaning strongly on younger voters.

“This time around there is uncertainty around the groups that Harris is relying on,” Professor Shaun Bowler, a political scientist at the University of California Riverside, says. “Younger voters tend not to vote as regularly as older ones – and we have no real data to go on for people voting in their first election.”

A much easier route to a Kamala Harris presidency would be to focus solely on Pennsylvania and its so-called ‘blue wall’ neighbours.

Pennsylvania already has a popular Democrat governor – Josh Shapiro – and John Fetterman’s mid-term victory in 2022 saw one of its Republican seats in the Senate turn blue.

“It seems more winnable than the other swing states,” says Professor Bowler. “Aside from a very popular Democrat governor, it also has an established South Asian community around Philadelphia.”

But Professor John Lapinski, director of elections at Sky’s partner network NBC News, warns: “No one knows who’s ahead in Pennsylvania, that’s why this race is so close.”

In 2016, the Republicans took Pennsylvania, Michigan, and Wisconsin – and Georgia, Arizona, and North Carolina.

Mr Trump’s popularity with white, non-educated voters could see him take the so-called ‘blue wall’ states in the north again this time.

Polling has also appeared to lure the Democrats into a false sense of security along the blue wall previously. Hillary Clinton was way ahead of Mr Trump in neighbouring Wisconsin in 2016 – but ended up losing there. More generally, polls have tended to overestimate levels of Democrat support in recent years.

Professor Lapinski adds: “Historically there’s been an underestimation of Trump. A lot of work has gone on to try to correct that in the polls.

“But while Harris is investing more money in most of these states than Trump – you can never underestimate his ability to turn out people.”

With the most campaign resources devoted to Pennsylvania on both sides, Mr Trump could be victorious there – meaning he would only have to secure Georgia and North Carolina to get to 270.

The attempted assassination attempt on him in July also happened at a rally in Butler, Pennsylvania. But despite the Trump campaign’s efforts to use it to his advantage, Mr Biden’s decision to step aside days later appears to have overshadowed it.

“It’s been such an eventful and fast-paced period that it has been pushed down the list of things people are thinking about,” Professor Bowler says.

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Georgia was one of the key states Mr Trump falsely claimed was “stolen” from him in 2020 and where he is still facing charges of election interference. Georgian Republicans are still trying to push through changes that would see officials allowed to refuse to certify the vote count there.

This is a tight path to victory for Mr Trump – as it puts him at exactly 270 Electoral College votes.

It also requires him to win Maine’s second congressional district – where there is no ‘winner takes all’ approach and Electoral College votes are divided between districts – but he won there in 2016 and 2020.

Winning in the South won’t ensure a Trump victory on its own. The former president would also have to win one northern swing state.

This could be Pennsylvania – as most campaign resources are being focused there – or the smallest in terms of Electoral College votes – Wisconsin.

Wisconsin borders Minnesota, however, where the Democrats’ vice president pick Tim Walz is governor. This means his campaign will have easy access to Wisconsin voters, making things “harder for Trump”, Professor Bowler says.

Ultimately, the Trump campaign could lose out on the big fight for Pennsylvania as long as it gets all four southern battlegrounds.

There is an alternative tiebreak scenario if the Democrats dominate in the North and the Republicans in the South.

Like in Maine, Nebraska’s Electoral College votes are awarded by Congressional district. Nebraska’s second Congressional district has emerged as its own swing over recent years.

If Ms Harris wins Pennsylvania, Wisconsin, and Michigan – but loses Nebraska second – she’ll be tied with Mr Trump on 269 votes.

Mr Biden won the district by a bigger margin than Pennsylvania, Michigan, and Wisconsin in 2020, however.

Nebraska Republicans tried to change voting rules to favour Mr Trump – but one state senator’s refusal to back the plan meant it failed to go through.

A tiebreak would also emerge if Mr Trump failed to hold Maine’s second district.

While neither of these scenarios is very likely, they effectively translate as a Trump victory. This is because the deciding vote would then go to the House – where Republicans have a majority.

Data reporting by Daniel Dunford, senior data journalist