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 either Democratic President Joe Biden or his Republican predecessor, Donald Trump.

Six of them - Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin – appear to be on a knife edge and probably hold the key to who will take the White House.

So both parties are campaigning intensively to win over undecided voters in these states.

Arizona

Mr Biden gained the presidency in 2020 with the support of the Grand Canyon State, which narrowly voted to back a Democratic 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. Arrivals at the US border have hit record highs while Mr Biden has been in office, presenting him with a major electoral headache. Border crossings have fallen in recent months but he has toughened his stance and made plans to enact border shutdowns when crossings surge.

Mr Trump has repeatedly attacked his rival’s record on immigration, and has 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 attempted to reinstate a 160-year-old near-total ban on terminating pregnancies. It’s become an explosive issue nationwide since 2022 when the US Supreme Court overturned a landmark ruling that gave women the constitutional right to abortion.

Alt text: BBC line graph titled “migrant numbers at the US-Mexico border: total number of encounter by US border patrols, per month” shows fluctuations in the month-to-month figures in the Trump and Biden administrations. The years 2018-2024 are shown, with significantly higher numbers since 2021, under Biden

Georgia

Our list of swing states closely matches the list of places in which Trump-backed Republican officials tried to thwart Mr Biden’s win in the 2020 election. In Georgia, alleged election interference has landed Mr Trump in one of his four criminal cases.

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

At any rate, it remains to be seen whether Mr Trump’s legal headaches will harm him at the ballot box. We may find out soon, now that his hush-money trial has ended with a guilty verdict.

At 33%, Georgia has one of the country’s highest proportions of African-Americans in its population, and it is believed that this demographic was instrumental in Mr Biden flipping the state in 2020. However, disillusionment has been reported among America’s black voters, with some saying not enough has been done to combat racial injustice or deliver on the economy.

Michigan

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, after a campaign was mounted by activists who want the US government to back a ceasefire in Gaza and 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 looks to be in jeopardy.

Mr 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”.

Nevada

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 Mr Trump has been enjoying a healthy lead over Mr Biden there. Both men 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 Mr Trump wins power again, he has vowed a return to an agenda of lower taxes across the board, and fewer regulations.

Getty Images A woman wears a hat and a face mask that reads "Biden Harris"Getty Images

Nevada voted Democrat in the last election, but its post-Covid recovery has been slower than elsewhere

Pennsylvania

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 recently 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”.

The state proved pivotal in the 2020 election, backing Mr Biden in his successful White House bid. He has a deep emotional connection to the working-class city of Scranton, where he grew up.

High inflation could hurt Mr Biden across the US as polling suggests it is giving voters an unfavorable view of the economy.

Mr Trump has sought to attack his adversary over persistently high prices. But he has his own challenges in Pennsylvania after a relatively strong showing there by his Republican primary rival Nikki Haley.

Wisconsin

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 these 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 an independent such as Robert F Kennedy Jr - who is fighting to get on the ballot in Wisconsin and other states - could harm the vote tallies for Mr Biden or Mr Trump.

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

Mr Biden recently pointed to a new Microsoft data centre heading to Wisconsin as evidence of how he was delivering new jobs; arguing that his predecessor had been unable to deliver on his pledges. (BBC)

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