It's the final day of campaigning for candidates, and polls suggest it is a historically close race

At a rally in North Carolina, Donald Trump announces that he intends to place 25% tariffs on all Mexican imports if they "don't stop" migrants from crossing the border

Kamala Harris is pushing her closing message with four stops throughout the battleground state of Pennsylvania - the pair will both hold rallies in Pittsburgh later today

The polls suggest the race is still exceptionally close, and more than 78 million people have already voted

If there was any question about where the White House is likely to be won and lost, today's election-eve campaign blitz should provide the answer.

Both presidential candidates are holding multiple rallies in Pennsylvania, thelargest of the seven swing states likely to determine the US election result.

The "blue wall" state is considered something of a must-win, particularly for Kamala Harris. No Democratic candidate has won an election without winning Pennsylvania since 1948.

Ms Harris will bring a roster of big celebrity names as she holds rallies in the state's two largest cities, while Donald Trump is hoping a female-focused event in a smaller city will improve his standing with women.

But Trump started his last day of campaigning further south, in Raleigh, North Carolina — the first of three states he'll visit today.

Tariffs on Mexico to stop immigrants

He used the Raleigh campaign stop to threaten Mexico with new tariffs if Mexican president Claudia Sheinbaum doesn't stop immigrants crossing the US border.

"I haven't met her, and I'm going to inform her on day one or sooner, that if they don't stop this onslaught of criminals and drugs coming into our country, I'm going to immediately impose a 25 per cent tariff on everything they send into the United States of America," he said.

"It's only got a 100 per cent chance of working, because if that doesn't work, I'll make it 50, and [if] that doesn't work, I'll make it 75. For the tough guys, and I'll make it 100," Trump said, adding Mexico would be forced to send soldiers to its southern borders with Guatemala and Belize to stop migrants.

Trump, who spoke for more than 90 minutes, said the pace of the campaign did not bother him.

"I don't even sleep," he declared.

He claimed Mexico was the US biggest trading partner, but it's not — it's second to Canada as the largest recipient of American goods and the second largest importer to America after China, which Trump has also threatened with a 60 per cent tariff during the campaign.

Economists have criticised Trump's reliance on tariffs, which could stoke inflation, affect global supply chains and lead to retaliation. Once again in Raleigh, Trump said that while voters rate the economy and the cost of living as their top issues, he thought controlling immigration was more important.

Polls still neck and neck

With election-day polling opening in less than 24 hours and early and mail-in voting well underway, the race is still too close to call.

In Pennsylvania, theFive Thirty Eight polling average for the statestill has the two candidates tied on 47.8 per cent each.

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Polls in the other swing states are also stubbornly tight. As of early November 4, local time, Trump was about 1 percentage point ahead in Nevada, North Carolina and Georgia, and has a 2.5 per cent lead in Arizona.

Harris was less than a percentage point ahead in Michigan and Wisconsin.

The Democratic camp was buoyed this week byshock Iowa pollingthat has them 3 percentage points ahead of Trump, whose team dismissed the result, which was in the margin of error, as "fake".

Celebrities rally around Harris

Back in Pennsylvania, pop singer Katy Perry and DJ D-Nice are hoping to give Harris a boost when they perform at her event in Pittsburgh on Monday.

Singers Lady Gaga and Ricky Martin, DJ Jazzy Jeff and TV star Oprah Winfrey are among the special guests appearing in Philadelphia.

Harris will use another rally in the state to try to capitalise on anger in the Puerto Rican community, aftera comedian at a Trump event called the US territory a "floating island of garbage".

That rally, at Allentown in Pennsylvania's east, will feature salsa singer Frankie Negron and rapper Fat Joe, who are both Puerto Rican.

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Trump is scheduled to appear in Pittsburgh around the same time as Harris, just a short walk from where her rally was set to be held.

The vice-president's team announced a venue change the day before her rally, reportedly because the Secret Service had raised safety concerns.

'That's why we don't trust the polls'

Trump is also continuing to campaign in the state, wherehe has been fuelling conspiracy theories about election theft.

"They are fighting so hard to steal this damn thing," he told a rally in the Pennsylvania borough of Lititz on Sunday, local time, without providing evidence.

Speaking to Trump supporters queuing up for his Pittsburgh rally, all mention the same thing: a deep distrust of the electoral process and of the polls that suggest Pennsylvania is neck-and-neck.

Some suspect America is in for a shock similar to 2016, when polls predicted a Hillary Clinton victory.

"I don't know one conservative who would respond to a poll," said Winkie Ilic, from the small borough of Seven Fields outside Pittsburgh.

"We, as a rule, don't. So that's why we don't trust the polls at all."

John Mueller, from Pittsburgh, also doesn't believe the state is tied.

"Not from what I'm seeing on the ground," he said. "A lot of Trump supporters, a lot of Trump yard signs and a lot of support."

The competing rallies mean the city's public schools have decided to close early, for fears of traffic chaos later in the day.

Trump will hold also hold a rally in Reading, Pennsylvania, as part of what's been billed the "Team Trump Women's Tour". It's part of an effort to close a significant gender gap that has Ms Harris well ahead of Trump with female voters, according to polls.

The speaker list includes Republican National Committee co-chair Lara Trump, influencer Emily Austin, and a lawyer who participated in Trump's reality TV show The Apprentice, Erin Elmore.

Trump will also hold election-eve events in the swing state of Michigan. (ABC News)