In a last-ditch effort to stave off the indictment of Donald J. Trump, a witness on Monday appeared before a Manhattan grand jury at the request of the former president’s lawyers, providing testimony that was aimed at undermining the credibility of the prosecution’s star witness.

The testimony came from a lawyer, Robert J. Costello, who was once a legal adviser to Michael D. Cohen, the crucial witness for the Manhattan district attorney’s office in its investigation of Mr. Trump. Mr. Cohen, Mr. Trump’s former fixer, has already spent hours testifying before the grand jury.

Mr. Costello and Mr. Cohen had a falling out a few years ago, and Mr. Costello’s grand jury appearance on Monday — at the request of Mr. Trump’s lawyers — was intended solely to attack Mr. Cohen, people with knowledge of the matter said.

Under New York law, a person who prosecutors expect will be indicted can request that a witness appear on his or her behalf. The final decision to hear the witness rests with the grand jury.

Mr. Costello confirmed his appearance in a text message but declined to elaborate.

Prosecutors had summoned Mr. Cohen to the courthouse where the grand jury meets, thinking he might be useful in rebutting Mr. Costello’s testimony. They did not call him into the grand jury on Monday, however, and it is unclear if Mr. Cohen could be called back later in the week.

“Mr. Cohen was available for over two hours today, but we are pleased to report Mr. Cohen was not needed,” his lawyer, Lanny J. Davis, said in a text message. He added: “Once again we repeat — the facts and documents speak for themselves. Facts do matter.”

The grand jury, which could indict Mr. Trump as soon as this week, has for weeks been hearing evidence about the former president’s involvement in a hush-money payment to a porn star.

Mr. Cohen paid $130,000 to buy the silence of the porn star, Stormy Daniels, who had said she had previously had an affair with Mr. Trump. Mr. Trump authorized the payment, according to Mr. Cohen, who pleaded guilty to federal charges involving the hush money in 2018. The federal prosecutors in that case substantiated Mr. Cohen’s claim that Mr. Trump directed that the payoff be made.

While president, Mr. Trump reimbursed Mr. Cohen, and the district attorney’s office has zeroed in on that reimbursement as a possible fraud.

The prosecutors are expected to charge Mr. Trump with overseeing the false recording of the reimbursements to Mr. Cohen as “legal expenses” in his company’s internal records.

Mr. Trump has denied all wrongdoing in the case and has said he never had an affair with Ms. Daniels. He has blasted the investigation as an effort by his political enemies to bring him down. He has also called Mr. Bragg, a Democrat who is Black, a “racist.”

Mr. Costello’s appearance is likely to coincide with attacks from Mr. Trump’s lawyers on Mr. Cohen’s credibility. At trial, Mr. Trump’s lawyers will be likely to point out that Mr. Cohen already pleaded guilty to federal crimes in 2018 stemming from the hush-money payment.

Prosecutors may counter that Mr. Cohen was lying on Mr. Trump’s behalf. In recent years, Mr. Cohen’s story has been consistent.

But Mr. Costello would argue that Mr. Cohen’s testimony cannot be trusted. Mr. Cohen and Mr. Costello were introduced in 2018, when Mr. Cohen was facing the federal investigation into the hush money and the men spent hours meeting and speaking by phone.

Mr. Costello, a Republican lawyer with ties to Mr. Trump’s legal team, offered to serve as a bridge between Mr. Cohen and the president’s lawyers. Mr. Costello once contacted one of Mr. Trump’s lawyers to raise the notion of a presidential pardon for Mr. Cohen.

But the pardon never came. Mr. Cohen never retained Mr. Costello. And their relationship worsened as Mr. Cohen broke from Mr. Trump.

Mr. Cohen eventually waived their attorney-client privilege, Mr. Costello has said, and when Mr. Costello’s law firm sent Mr. Cohen a bill, he refused to pay.

Nate Schweber, William K. Rashbaum and Jonah E. Bromwich contributed reporting.