Ballot boxes have been set on fire with incendiary devices amid a keen uptake of early voting in the US election.

Some 46 million Americans have voted already, according to the Election Lab at the University of Florida, among them President Joe Biden who cast his ballot on Tuesday (AEDT).

Biden waited in line for 40 minutes, chatting with others, not far from his home town outside Wilmington, Delaware.

The room erupted in cheers as he presented his identification and was announced by an election official as “Joseph Biden now voting”.

Americans have made the most of early voting, especially in battleground states such as Georgia and North Carolina, which have broken turnout records.

Polls show the presidential election is essentially a dead heat between Republican candidate Donald Trump and his Democratic rival Kamala Harris.

Early on Monday (US time), there were reports of two ballot drop boxes on fire, which police believed were connected.

In the first instance, a ballot drop box in Portland, Oregon, was in flames. Suppressant inside the box protected nearly all the ballots, with only three suffering damage.

Those voters will be contacted to receive a replacement ballot.

Oregon Secretary of State LaVonne Griffin-Valade said an attack on a ballot box was an “attack on [US] democracy and completely unacceptable” .

“Whatever the motivation behind this incident, there is no justification for any attempt to disenfranchise voters,” she said.

About half an hour later, a second ballot box was alight in Vancouver, Washington, with hundreds of ballots damaged.

A suspicious vehicle believed to be linked to a ballot drop box fire in Portland, Oregon. Photo: Portland Police Bureau

On Tuesday (AEDT), Trump appealed to religious voters in the southern swing state of Georgia as his campaign distanced itself from racist remarks by allies that could alienate key voter groups.

In Georgia, in-person early voting ahead of Election Day is expected to total up to 70 per cent of ballots.

Trump faces pressure over his rally in New York on Sunday, where a comedian called Puerto Rico a “floating island of garbage,” prompting a backlash from Latino celebrities and criticism from both Republican and Democratic politicians.

The Trump campaign has said the joke did not reflect their views.

Vice President Kamala Harris said Trump “fans the fuel of hate and division, and that’s why people are exhausted with him”.

Later, Trump rejected Harris’ contention that he is a fascist, telling thousands at a rally in Atlanta: “I’m not a Nazi. I’m the opposite of a Nazi.”

Earlier, Trump had attacked Harris by falsely claiming she was letting “some of the worst criminals in the world into our country”.

“Kamala has imported criminal migrants from prisons and jails, from insane asylums and mental institutions, from all around the world – from Venezuela to the Congo,” he said.

Georgia is one of seven competitive swing states expected to play a decisive role in an election that wraps up in just eight days. National polls including the Reuters/Ipsos poll and surveys in swing states show the two candidates in a close match.

At stake in next week’s presiential election is stewardship of the world’s most powerful country. Harris and Trump diverge on support for Ukraine and NATO, tariffs that could trigger trade wars, abortion rights, taxes and basic democratic principles.

-with AAP