Days before Donald J. Trump became president in early 2017, a handful of advisers, officials and allies descended on his office at Trump Tower: the F.B.I. director, a future secretary of state, his soon-to-be chief of staff — and the publisher of The National Enquirer.

The publisher, David Pecker, may have seemed out of place, but he had just performed an indispensable and confidential service to the Trump campaign: He had paid off a Playboy model, Karen McDougal, who said she had an affair with Mr. Trump, and a doorman who had heard that Mr. Trump had fathered a child out of wedlock. The future president, triumphant, thanked Mr. Pecker for his service.

That remarkable scene was private until Thursday, when Mr. Pecker recounted it to jurors in Mr. Trump’s Manhattan criminal trial. He described in vivid detail how Mr. Trump laid bare their effort to buy and bury damaging stories that could have derailed Mr. Trump’s campaign — a plot at the center of the case.

“He said, ‘I want to thank you for handling the McDougal situation,’ and then he also said, ‘I wanted to thank you for the doorman situation,’” Mr. Pecker testified Thursday. “He was thanking me for buying them and for not publishing any of the stories and helping the way I did. He said that the stories could be very embarrassing.”

Mr. Trump also asked after Ms. McDougal: “How’s our girl?” Mr. Pecker said he replied, “She’s cool. She’s very quiet. No issues.”

Mr. Pecker’s testimony was a stunning moment in the first criminal trial of an American president. Coming on his third day on the stand, it underscored how Mr. Pecker’s support in 2016 has now come back to haunt Mr. Trump in 2024.

The former publisher transported jurors into the room at Trump Tower that day in January 2017. He was there alongside four people who would become key figures in the Trump presidency: Sean Spicer, press secretary; Reince Priebus, chief of staff; Mike Pompeo, C.I.A. director and later secretary of state; and James Comey, the F.B.I. director whom Mr. Trump would ultimately fire.

Mr. Trump introduced Mr. Pecker to the men, and then added slyly that Mr. Pecker probably “knows more than anyone else in this room.”

“It was a joke,” Mr. Pecker testified. “Unfortunately, they didn’t laugh.” (On Thursday, however, Mr. Trump chuckled at the defense table.)

Mr. Pecker, who delivered hours of testimony on Thursday, described how he had helped quash three scandalous stories about Mr. Trump, including by setting in motion a hush-money deal with a porn star, Stormy Daniels. That payment is central to the prosecution’s case against Mr. Trump, and Mr. Pecker’s testimony illuminated the deal to a captivated jury.

The payment came about after Mr. Pecker relayed Ms. Daniels’s story to Mr. Trump’s lawyer and fixer at the time, Michael D. Cohen, and urged him to pay her off. Mr. Cohen ultimately did, to the tune of $130,000.

He warned Mr. Cohen that if he failed to keep Ms. Daniels quiet, Mr. Trump would be furious: “The boss is going to be very angry with you,” he told Mr. Cohen.

Mr. Pecker’s testimony, which kept many jurors rapt as Mr. Trump shifted and slumped in his chair, spoke to a central theme in the prosecution’s case. Mr. Pecker, prosecutors contend, joined a three-man conspiracy with Mr. Trump and Mr. Cohen, hatching a plot to hide damaging stories from the American people.

Mr. Pecker introduced the jury to the practice known as “catch and kill” — buying the rights to a story with no intention of publishing it — a dark art in the world of supermarket tabloids. The National Enquirer used the tactic to silence Ms. McDougal and the doorman.

He took jurors behind the scenes of the shady deal-making, detailing how he had bought Ms. McDougal’s story for $150,000 and disguised the payment as a deal for other services, including writing columns. Those supposed services, he acknowledged, were camouflage for an illegal donation to Mr. Trump’s campaign.

In a powerful moment for the prosecution, Mr. Pecker acknowledged a clear-cut motive for keeping the model’s story under wraps: protecting Mr. Trump’s chance of winning the White House.

“We didn’t want the story to embarrass Mr. Trump or embarrass or hurt the campaign,” Mr. Pecker testified.

He also acknowledged that it is unlawful for a corporation to spend money that way to influence the election, another pivotal moment in the early days of the trial.

Mr. Pecker explained to the jury that he had learned Ms. Daniels was looking to sell her story as Mr. Trump’s campaign was reeling from the publication of the “Access Hollywood” recording, in which Mr. Trump boasted of grabbing women by their genitals.

That tape, he said, “was very embarrassing, very damaging to the campaign.”

But he had already shelled out the $150,000 to Ms. McDougal, and he balked at paying Ms. Daniels, leaving it to Mr. Cohen to strike the $130,000 hush-money deal. “After paying out the doorman, after paying out Karen McDougal, we’re not paying out any more moneys,” Mr. Pecker recalled telling Mr. Cohen.

The prosecutors, from the Manhattan district attorney’s office, accused Mr. Trump of falsifying business records when reimbursing Mr. Cohen for the $130,000 payment and charged the former president with 34 felonies — one for each check, ledger and invoice related to the repayment.

Mr. Trump denies that he and Ms. Daniels had sex and has said he did nothing wrong. If convicted, he could receive probation, or up to four years in prison.

Although Mr. Pecker was not directly involved in creating the false records, his story was essential to the prosecution case.

As he described his interactions with the district attorney’s office, Mr. Trump became animated, shaking his head several times in stern disapproval.

Under cross-examination from a lawyer for Mr. Trump, the tabloid publisher acknowledged that it was a standard practice for his publication to buy stories as leverage for access and interviews with celebrities. He also admitted giving Mr. Trump a heads-up about negative stories for years before he ran for president.

Under questioning from prosecutors, Mr. Pecker spent much of his time on the stand detailing the deal with Ms. McDougal, whose lawyer brought the story to The National Enquirer, which then vetted the account.

Ms. McDougal, he said, was happy to stay quiet.

“She said she didn’t want to be the next Monica Lewinsky,” he explained.

Mr. Pecker alerted Mr. Cohen, who then pressed the tabloid to buy her story. When Mr. Pecker expressed concern about who would pay the $150,000 — noting that “this is a very, very large purchase” — Mr. Cohen reassured him. He said, “The boss will take care of it.”

And at one point, Mr. Trump and Mr. Pecker spoke directly about the deal, the former publisher testified. Mr. Trump, he said, called Ms. McDougal “a nice girl,” leading Mr. Pecker to believe that the candidate “knew who she was.”

Mr. Trump was reluctant to pay, and soon Mr. Cohen was waffling as well — instead, Mr. Cohen persuaded Mr. Pecker to have his company make the payment. He assured him that the boss would pay Mr. Pecker back.

Ultimately, when Mr. Cohen created a shell company to repay the tabloid, it was Mr. Pecker who got cold feet amid concerns about the legality of the arrangement.

“The deal is off,” Mr. Pecker said he told Mr. Cohen.

That was not the end of the saga. Just days before the election, The Wall Street Journal published a story revealing The National Enquirer’s deal with Ms. McDougal. This prompted an irate call to Mr. Pecker.

“Donald Trump was very upset,” Mr. Pecker said Thursday, describing how the candidate asked, “How could this happen? I thought you had this under control.” Mr. Trump, he said, blamed The National Enquirer for leaking the story. Then he hung up.

“He didn’t say goodbye, which was very unusual,” Mr. Pecker recalled.

The story did not change the outcome of the election.

After Mr. Trump’s victory, Mr. Pecker encountered him at the Trump Tower meeting in January 2017 and then in July of that year at the White House.

“Mr. Trump asked me to join him in the walk from the Oval Office to the dining area,” Mr. Pecker recalled, and on the walk, Mr. Trump posed a question: “How is Karen doing?”

Mr. Pecker replied: “She’s doing well. She’s quiet. Everything’s going good.”

Maggie Haberman, Kate Christobek and Wesley Parnell contributed reporting.