Washington DC - Kamala Harris will unveil plans Wednesday to help small US businesses, her campaign said as the Democratic presidential candidate fleshes out her economic policy ahead of a critical debate with Donald Trump.

The vice president will propose tax breaks and cutting red tape for smaller firms if elected in November, she will tell a campaign event in the state of New Hampshire.

Harris has ridden a wave of enthusiasm since replacing President Joe Biden as Democratic candidate, pledging an "opportunity economy", but has faced criticism for detailing how she'd do that.

A Harris campaign official said that she would "propose a dramatic tenfold expansion of tax relief for starting a small business and set a goal of 25 million new small business applications in her first term."

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The plan would increase the tax deduction small businesses can make for startup costs from $5,000 to $50,000.

Her speech in Portsmouth, New Hampshire will come hours before Republican former president Trump takes part in a town hall with Fox News in Harrisburg in the crucial swing state of Pennsylvania.

The two candidates are battling it out over taxes in particular, with Trump accusing Harris of copying a policy of ending taxes on tips for service workers and pledging tax cuts across the board.

Harris is meanwhile pushing to raise taxes on corporations and richer households.

Trump and Harris are due to hold their first presidential debate in Philadelphia, Pennsylvania on September 10.