About 240 million people are eligible to vote in this year’s US election, but only a relatively small number of them are likely to settle the question of who becomes the next president.

Experts believe there are only a handful of states that could plausibly be won by Kamala Harris or Donald Trump, for the Democrats or Republicans respectively.

Seven of them - Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin - hold the keys to the White House.

Now, in the final months of the election, both campaigns are at full throttle to win over undecided voters in these states.

The Democrats gained the presidency in 2020 with the support of the Grand Canyon State, which narrowly voted to back their party's candidate for the first time since the 1990s.

This state borders Mexico for hundreds of miles, and has become a focal point of the nation’s immigration debate.

Border crossings have fallen in recent months from record highs, which have made the issue near the top of voters' concerns.

Trump has repeatedly attacked Ms Harris’s record on immigration, because she was given a role by President Joe Biden in trying to ease the border crisis.

He has also vowed to carry out “the largest deportation operation” in US history if he regains the presidency.

Arizona has also hosted a bitter row over abortion access, after state Republicans tried unsuccessfully to reinstate a 160-year-old near-total ban on terminating pregnancies.

The issue has become even more polarising since 2022 when the US Supreme Court overturned a landmark ruling that gave women a constitutional right to abortion.

Our list of swing states closely matches the list of places where Trump-backed Republican officials tried to thwart Mr Biden’s win in the 2020 election.

In Georgia's Fulton County, alleged election interference has landed Mr Trump in one of his four criminal prosecutions (Trump has been convicted in one case, while the remaining are ongoing).

He and 18 others are accused of conspiring to overturn his narrow defeat to Mr Biden in the state. Trump denies any wrongdoing, and the case looks increasingly unlikely to be heard in court before the election.

A third of Georgia's population is African-American, one of the country’s largest proportions of black residents, and it is believed that this demographic was instrumental in Mr Biden flipping the state in 2020.

Some disillusionment with Mr Biden had been reported among America’s black voters, but the Harris campaign hopes to animate this constituency.

The Great Lakes state has picked the winning presidential candidate in the last two elections. Despite backing Mr Biden in 2020, it has become symbolic of a nationwide backlash over the president’s support for Israel during that country’s war in Gaza.

During Michigan’s Democratic primary contest in February, more than 100,000 voters chose the “uncommitted” option on their ballots, part of a campaign mounted by activists who want the US government to halt its military aid to Israel.

Notably, Michigan has the country’s largest proportion of Arab-Americans - a demographic whose support for Mr Biden was in jeopardy. But Ms Harris has taken a harder tone on Israel, and some Gaza protesters have told the BBC they hope she will be more sympathetic to their cause.

Trump has highlighted the state’s significance in his potential path to victory. Commenting on events in the Middle East, he has called on Israel to finish its campaign over Hamas in Gaza, but "get it over with fast”.

The Silver State has voted Democrat for the last several elections, but there are signs of a possible turnaround by the Republicans.

Recent averages published by poll-tracking firm 538 indicate that polls once showed Trump enjoying a wide lead over Mr Biden, but that advantage has shrunk since Ms Harris became the top of the ticket. Democrats had hoped a candidate with greater appeal to younger and more diverse voters would close the gap.

Both candidates are vying to win the state’s sizeable Latino population.

Despite the fact that the US economy has shown strong growth and jobs creation since Mr Biden took the presidency, the post-Covid recovery has been slower in Nevada than elsewhere.

At 5.1%, the state has the highest unemployment rate in the country, after California and the District of Columbia, according to the latest US government statistics.

If Trump comes to power again, he has vowed a return to an agenda of lower taxes across the board, and fewer regulations.

Polls appear to have tightened in the Tar Heel State after Ms Harris assumed her place at the top of the Democratic ticket, and some analysts now call it a "toss-up".

That has perhaps been reflected in Trump's choice to go there for his first outdoor rally since an attempt was made on his life in July.

“This state is a very, very big state to win," he told the crowd.

For their part, the Democrats decided to give a platform to the state's governor, Roy Cooper, on the final night of their party convention.

North Carolina borders Georgia, and shares some of its top electoral concerns, as well as those in Arizona, another Sun Belt state.

Trump carried North Carolina in 2020 but did so by just over 70,000 votes, which has further buoyed Democrats' hopes that this "purple" state (one that could vote red or blue) could be winnable in this election year.

Both sides have also been campaigning doggedly in the Keystone state, which is where Donald Trump survived the assassination attempt.

The state proved pivotal in the 2020 election, backing Mr Biden. He has often spoken of his connection to the working-class city of Scranton, where he grew up.

The economy is a a top issue here, as well as many other places. Inflation spiked across the country under the Biden administration, before gradually reducing.

Pennsylvanians are far from unique among Americans for feeling cost-of-living pressures as a result of inflation. But the price of groceries has risen faster in their state than in any other, according to market intelligence provider Datasembly.

The BBC has previously reported on how people were struggling to cope in Erie - a bellwether county for the rest of Pennsylvania, where as many as one in eight people are considered “food insecure”.

High inflation could hurt Ms Harris across the US as polling suggests it is giving voters an unfavourable view of the economy.

Trump has sought to attack her by pinning her to the Biden economy.

The Badger State also picked the winning presidential candidate in both 2016 and 2020, by a margin of little more than 20,000 votes each time.

Pundits have suggested it is in marginal states like this one where an impact could be made by third-party candidates who are campaigning against the policies of the big two candidates.

Polling has indicated that a sizeable show of support for independent candidate Robert F Kennedy Jr could harm the vote tallies for Ms Harris or Trump. Democrats have been fighting to have Green Party candidate Jill Stein removed from the ballot here, saying the party did not comply with state election laws, and has also filed an elections complaint against Cornel West, a left-leaning academic.

Trump has described that state as “really important… if we win Wisconsin, we win the whole thing”. The summer Republican National Convention was held in Milwaukee.

Ms Harris was rallying in that same city when her party's own convention formally nominated her as the Democratic presidential candidate, appearing via a live feed.