Voters across 50 states are preparing to cast their ballots after a bitterly contested US election campaign, which will see Donald Trump or Kamala Harris become president.

In the last few hours, both candidates have been giving their final pitches. “The momentum is on our side,” Ms Harris told a crowd in Philadelphia that chanted back, “We will win”.

“Tonight, then, we finish as we started: with optimism, with energy, with joy,” she said, while enjoying the support of celebrity endorsements on the day from Katy Perry, Lady Gaga and Jon Bon Jovi.

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In contrast, Mr Trump ended his campaign in Michigan, repeating key messages about the economy and immigration.

A handful of states will play a crucial role in determining the outcome of the election. Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin hold the keys to the White House.

To become president, the winning candidate needs 270 electoral votes or more, with each state carrying a different number of votes.

But the focus has been on Pennsylvania which carries 19 electoral votes, the most of all the swing states.

It has been a remarkable journey for both candidates – with Mr Trump surviving two assassination attempts and Ms Harris not even originally in the running.

For more than a year, the 2024 presidential race seemed destined for a rematch between Joe Biden and Mr Trump – but a disastrous TV debate by Mr Biden eventually forced him to withdraw from the ticket.

The Democratic party’s decision to replace Mr Biden with his vice president transformed the race and shifted polls in Ms Harris’s favour. But only just.

Many polls are too tight to call and, with a large margin of error, most experts are refusing to predict the outcome.

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Later, attention will turn to those battleground states including Georgia, which is among the first polls to close at 7pm local time (midnight UK time).

State election officials told Sky News they could have a result as early as 10pm (3am UK time).

Even so, it could be several days before the US has a definitive result.

Voters are not just selecting a president. In addition, 10 states will hold abortion-related ballots, half of which would overturn existing restrictions.

Predict who you think will win in each swing state and we’ll tell you who the president will be if you’re right.

Tonight, Sky News will have access to the most comprehensive exit poll and vote-counting results from every state, county and demographic across America through its US-partner network NBC.

You can find out more about Sky News’ coverage here.

Up to 4,000 people voting overseas in the US election are having their ballots challenged in Pennsylvania, a key battleground state, according to the American Civil Liberties Union (ACLU).

They include Selma Aldi, 47, from Camden in north London who received a letter on Sunday explaining that her ballot in the US presidential election is at risk of being rejected.

“It was a shock,” she said. “It was terrifying to be targeted, to potentially lose a right that I hold as very important. It’s even a feeling that someone is questioning my identity.”

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The trainee GP, who grew up in Hershey, Pennsylvania left America in 2000 but has voted via absentee ballot in every US presidential election since.

A letter from election officials in Dauphin County outlines the legal challenge. It reads: “The applicant is not registered to vote and therefore is not eligible to vote in Pennsylvania.

“Under Pennsylvania law, it is a felony to permit any person to vote who is not registered.”

A hearing on the legal challenge is scheduled for Friday, in which Ms Aldi can respond.

Around 2.8 million US citizens living abroad are entitled to vote in the election, no matter where they are on polling day.

But each state has different procedures and rules on how election paperwork can be sent and received.

But Ari Savitzky, senior staff Attorney at the ACLU said “any attempt to challenge [voters] eligibility is a clear violation of their rights”.

He told Sky News: “Between 3,000 and 4,000 challenges have been filed in Pennsylvania to the absentee ballots of US citizens living abroad.

“For decades, federal law has guaranteed the right of US citizens living abroad to vote in federal elections at their last US residence.

“In addition to being legally baseless, these challenges are an abuse to voters and to election administrators.”

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Deborah Hinchey from another voting rights group, All Voting is Local, said: “Election deniers across Pennsylvania have submitted thousands of mass challenges to overseas voters.

“They want to block as many ballots as possible and silence our voices… but these baseless challenges have failed before and the proper checks and balances are in place to make sure they’ll fail again,” she added.

Tonight, Sky News will have access to the most comprehensive exit poll and vote-counting results from every state, county and demographic across America through its US-partner network NBC.

You can find out more about Sky News’ coverage here.

Elon Musk can keep giving away $1m to voters in battleground states, a judge has ruled – as a lawyer admitted the winners aren’t chosen randomly.

Musk – a supporter of Republican candidate Donald Trump – launched the giveaways last month via America PAC, his political action committee (PAC).

He has already handed out $16m in the scheme, which is open to registered voters in seven key battleground states – Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin – who sign a petition pledging to support free speech and gun rights.

On Monday, Pennsylvania Judge Angelo Foglietta ruled the giveaways could carry on, rejecting a district attorney’s request that he shut it down because it allegedly violated state election law.

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Philadelphia District Attorney Larry Krasner, a Democrat, said it was “a political marketing masquerading as a lottery”, adding “That’s what it is. A grift.”

Judge Foglietta did not explain his ruling on the matter but Chris Gober, a lawyer for America PAC, had argued the winners are not chosen by chance and are instead hand-picked based on who would be the best spokespeople for the group – despite Musk’s assertion that they would be chosen randomly.

Mr Gober said the final two winners before Tuesday’s presidential election will be in Arizona on Monday and Michigan on Tuesday.

He said the recipients “are not chosen by chance”, adding: “We know exactly who will be announced as the recipient today and tomorrow.”

America PAC director Chris Young said recipients are vetted ahead of time to “feel out their personality, (and) make sure they were someone whose values aligned” with the group.

In closing arguments, Musk’s legal team said it was “core political speech” as anyone taking part had to sign a petition endorsing the US Constitution.

Given there will be no more Pennsylvania winners before the programme ends, Musk’s lawyers said any legal bid to stop it under Pennsylvania law was irrelevant.

Launching the plan in the state on 19 October, Musk said they would be “awarding a million dollars randomly to people who have signed the petition every day from now until the election.”

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The following day, he said in a social media post shown in court that anyone signing the petition had “a daily chance of winning one million dollars!”

Musk did not attend the hearing.