Washington DC - Republican presidential candidate Donald Trump said late Friday that he had agreed to debate his Democratic opponent, Vice President Kamala Harris, on Fox News on September 4.

"I have agreed with FoxNews to debate Kamala Harris on Wednesday, September 4th," Trump wrote on his Truth Social platform.

It was not immediately clear if Harris had agreed to the debate.

Trump's post came hours after the vice president had secured the Democratic Party's nomination as standard bearer in the November presidential election.

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Trump's White House bid was turned upside down last month when 81-year-old President Joe Biden, facing growing concerns about his age and lagging polling numbers, abandoned his re-election bid and backed Harris.

His decision to bow out followed a disastrous debate performance against Trump in June on CNN.

A second Trump-Biden debate had been slated for September 10 on ABC.

That was expected to go ahead as planned, with Harris replacing Biden, but a spokesperson for Trump last week said it would be "inappropriate" to schedule a debate before Harris was formally the Democratic nominee.

The 78-year-old Republican nominee had previously said he would not debate Harris, who is nearly two decades his junior, departing from political tradition.

The former prosecutor and ex-California attorney general last month dared Trump to debate her face-to-face.

"Well Donald, I do hope you'll reconsider to meet me on the debate stage, because as the saying goes, if you've got something to say, say it to my face," Harris said at a campaign rally in Atlanta.

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In his post, Trump said the debate will be held in Pennsylvania, and moderated by Bret Baier and Martha MacCallum in front of a full audience.

Fox News confirmed that the debate would have "spectators" and follow similar rules to the June 27 CNN debate between Trump and Biden.

The news channel said it had invited Trump and Harris to participate in a debate in Pennsylvania in September.

"I look forward to meeting and debating Kamala Harris on September 4th," Trump said, adding the date was "convenient and appropriate" as it comes just before the September 6 start of early voting for the presidential election.