Former President Donald J. Trump said on Friday that he agreed to debate Vice President Kamala Harris on Sept. 4 in an event moderated by Fox News. If it happens, that would be the second presidential debate this election cycle and the first between Mr. Trump and the new Democratic candidate.

According to Mr. Trump’s post on his social media site, the debate is to take place at a to-be-determined location in Pennsylvania, the must-win battleground state that could help decide the election. Fox News anchors Bret Baier and Martha MacCallum would moderate.

It was unclear early Saturday if Ms. Harris had agreed to the debate and its terms. Representatives for her campaign did not respond to requests for comment. A spokeswoman for Fox News also did not respond to questions.

Mr. Trump said that the Fox News debate would have a live audience; the previous debate between President Biden and Mr. Trump took place in an empty venue. Though the campaigns agreed to the format of the first debate, Mr. Trump previously bemoaned that there was no crowd.

Mr. Trump also said that he is “totally prepared to accept” Ms. Harris as the Democrats’ new candidate. Since her campaign suddenly took shape after Mr. Biden dropped out of the race, he has characterized her ascendancy as a “coup” within the Democratic Party. The former president complained about the shake-up among his opponents in his debate announcement.

“I spent Hundreds of Millions of Dollars, Time, and Effort fighting Joe, and when I won the Debate, they threw a new Candidate into the ring,” Mr. Trump said on his social media site on Friday, adding he hopes to tie Ms. Harris to Mr. Biden’s policies.