Washington DC - Kamala Harris's campaign branded Donald Trump "scared" Saturday after he proposed to change the presidential debate schedule ahead of a rally in Georgia where he will try to halt the vice president's surging momentum in her bid to become America's first woman president.

In an overnight post on his Truth Social app, Trump said he was willing to debate Harris on the conservative Fox News network on September 4, while declining to participate in a previously scheduled debate on ABC.

Trump pitched the idea before going to a rally in Atlanta, where he will gather supporters in the same arena where Harris addressed an excited crowd of some 10,000 just this Tuesday.

Trump said he had "agreed" to the debate plan with Fox. And he said it would take place in Pennsylvania – a crucial swing state – in front of a live audience.

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The Harris campaign dismissed this as "games."

"Donald Trump is running scared and trying to back out of the debate he already agreed to and running straight to Fox News to bail him out," Harris's campaign communications director Michael Tyler said in a statement.

"He needs to... show up to the debate he already committed to on Sept 10."

The proposal to confront Harris on a network that has long supported him was the latest Trump effort to recapture the initiative in a campaign that had been entirely focused on a rematch against 81-year-old Joe Biden, until he dramatically dropped his reelection bid on July 21.

Since then, Harris (59) has reenergized the Democratic base almost overnight.

She has raked in donations, reassembled the team behind Barack Obama's two historic election victories, and neutralized the solid lead that 78-year-old Trump had built against Biden in opinion polls.

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On Friday, she secured the official Democratic nomination, backed by near unanimous party support.

Harris is due imminently to announce her vice presidential pick, with the popular governor of key state Pennsylvania, Josh Shapiro, a frontrunner. On Tuesday, she is set to launch a countrywide tour with her yet-to-be-named running mate.