Donald J. Trump, having failed to fend off a criminal trial in Manhattan that begins on Monday, said that he planned to testify in the case stemming from a hush-money payment to a porn star.

Taking questions Friday from reporters at his Mar-a-Lago estate in Palm Beach, Fla., Mr. Trump, when asked whether he would take the stand, responded that he would.

“I’m testifying. I tell the truth,” he said, standing just off a sunny patio of the private club with Speaker Mike Johnson behind him. “I mean, all I can do is tell the truth. And the truth is that there’s no case. They have no case.”

A spokeswoman for the Manhattan district attorney’s office, which has charged Mr. Trump with 34 felonies, declined to comment on his remarks.

Despite Mr. Trump’s comments, it is far from a sure thing that he will testify. Instead, his comments initiate a familiar two-step: It will not be clear whether the former president will take the stand until the moment he actually does.

Mr. Trump will most likely wait to see whether the prosecution presents a strong case — and whether the judge presiding over the trial plans to restrict prosecutors’ efforts to cross-examine him, according to people with knowledge of his planning.

In past cases, Mr. Trump has wavered after saying that he would testify, including during his civil fraud trial last year, when he canceled his defense testimony the day before he was scheduled to take the stand.

When he was called to testify by the New York attorney general’s office, which filed the case, it did not go well. The judge in the case, who found Mr. Trump liable for conspiring to inflate his net worth, criticized the former president for not answering directly and questioned his credibility.

Testifying in a criminal case would be even riskier. In the trial scheduled to start next week, Mr. Trump is for the first time facing the threat of criminal conviction. He will be at a disadvantage with a jury in Manhattan, a heavily Democratic county.

“Jury selection is largely luck,” Mr. Trump said in his remarks on Friday. “It depends who you get.”

The district attorney, Alvin L. Bragg, accused Mr. Trump of coordinating the hush-money payment to the porn star, Stormy Daniels, during the 2016 election. The $130,000 payment bought her silence as she was shopping a story about a sexual encounter with Mr. Trump.

Mr. Trump was charged with falsifying documents to cover up that payment.

Mr. Trump denies the charges, and having had sex with Ms. Daniels. He has cast Mr. Bragg’s case as a politically motivated witch hunt.

Jury selection is scheduled to begin Monday.