Washington DC - Kamala Harris and Donald Trump return to the campaign trail Wednesday with new polls showing the White House race remains on a knife edge less than a week before their crucial first debate.

Vice President Harris, who has revived Democratic hopes with just over two months until the November 5 vote, will be unveiling proposed tax breaks for small businesses as she fleshes out her economic policy during a visit to New Hampshire.

Trump meanwhile takes part in a pre-recorded town hall event on conservative Fox News in the battleground state of Pennsylvania, after a notable absence by the Republican from the trail over Labor Day weekend, traditionally seen as the start of the final sprint.

The rivals are getting in some last-minute campaigning ahead of the presidential debate on Tuesday, when millions of American voters will finally get to see them do battle in person on primetime television.

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"She's had her honeymoon period. People are learning who she is," Trump told a local New Hampshire radio station on Wednesday ahead of Harris's visit.

"We can't go another four years with a dumb president."

Harris has upended the race since President Joe Biden dramatically quit as the Democratic candidate after the 81-year-old's disastrous performance in a debate against Trump sparked fears about his age and mental acuity.

However, now the oldest presidential nominee in US history, Trump faces an entirely new challenge from Harris, who, at 59, is touting herself as a breath of fresh air.

A CNN poll released Wednesday showed that the race remains nail-bitingly close in six key battleground states that will likely decide the next occupant of the Oval Office.

Harris holds the edge against Trump over likely voters in the "Rust Belt" states of Wisconsin and Michigan by 50% to 44% and 48% to 43%, respectively. Trump, meanwhile, has the edge in Arizona by 49% to 44%.

But the race is essentially tied in three other states – and most critically in Pennsylvania, the biggest prize in terms of votes in the electoral college system.

Harris and Trump are tied at 47% in up-for-grabs Pennsylvania, while in Georgia and Nevada, 48% of voters back Harris to 47% for Trump.

Biden won all six states when he defeated Trump in 2020.

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The findings come a day after a USA Today/Suffolk University poll found Harris leading Trump by 48% to 43% – with double-digit gains with crucial groups, including Hispanic and Black voters and younger people, it found.

In her campaign event in Portsmouth, New Hampshire, Harris is set to propose a tenfold increase in tax breaks and cutting red tape for smaller firms.

Harris has pledged to tax corporations and the rich instead, contrasting her policy with Trump's pledge to cut taxes across the board, but has faced criticism for not setting out her policies more fully.

Trump has meanwhile accused Harris of copying a policy of ending taxes on tips for service industry workers.

He will take part in a town hall with Fox News host and close ally Sean Hannity. It will be taped and broadcast at 9:00 PM local time.

The campaigning continues for the rest of the week, with Harris traveling to Pittsburgh, Pennsylvania, on Thursday, then staying in the state ahead of the debate in Philadelphia on Tuesday.

She rallied with Biden in a joint appearance in Pittsburgh on Monday, and the outgoing president will whip up support for her – and his own legacy – with events in Wisconsin and Michigan on Thursday and Friday.