Advisers to former President Donald J. Trump said they would not commit to another debate, one they had already agreed to participate in, now that the Democrats have changed candidates from President Biden to Vice President Kamala Harris.

Mr. Biden and Mr. Trump had agreed to two general election debates, the first of which took place on June 27. Mr. Biden’s performance was so calamitous that it began a four-week drumbeat toward his departure from the race.

The two men agreed months ago to a second debate, to be held on Sept. 10, and hosted by ABC News. Mr. Trump complained at the time that they should have even more.

But Mr. Biden announced on Sunday that he was dropping out of the race, and within 48 hours, the entire Democratic Party had coalesced around Ms. Harris as the nominee to compete against Mr. Trump, the Republican nominee for the third straight presidential election. The only major Democrat yet to endorse Ms. Harris — former President Barack Obama — is expected to imminently. Ms. Harris’s team is in the early stages of vetting potential running mates.

Nonetheless, in a statement issued late on Thursday, Mr. Trump’s communications director, Steven Cheung, maintained that there was “continued political chaos surrounding” Mr. Biden and the Democrats, so “general election debate details cannot be finalized until Democrats formally decide on their nominee.”

The statement insisted that many in the Democratic Party — including Mr. Obama — think that Ms. Harris cannot beat Mr. Trump and that “they are still holding out for someone ‘better.’” The statement added: “Therefore, it would be inappropriate to schedule things with Harris because Democrats very well could still change their minds.”