Donald J. Trump had a dismal day in court on Tuesday as the judge presiding over his criminal trial told a defense lawyer he was “losing all credibility” and a key witness pulled back the curtain to expose what prosecutors called a conspiracy to influence the 2016 election.

The witness was David Pecker, longtime publisher of The National Enquirer, and he transported jurors back to a crucial 2015 meeting with Mr. Trump and his fixer at Trump Tower in Midtown Manhattan.

Prosecutors called it the “Trump Tower conspiracy,” arguing that Mr. Pecker, Mr. Trump and Michael D. Cohen, who was then Mr. Trump’s personal lawyer and fixer, hatched a plot at the meeting to conceal sex scandals looming over Mr. Trump’s campaign.

Their effort led Mr. Pecker’s tabloids to buy and bury two damaging stories about Mr. Trump. Mr. Cohen also purchased the silence of a porn star, a deal at the heart of the case against the former president.

In gripping testimony Tuesday, Mr. Pecker took the jurors inside the meeting, recalling how Mr. Cohen and Mr. Trump asked what he and his magazines — fixtures of American supermarket checkout lanes — could do “to help the campaign.” The account bolstered the prosecution’s argument that the men were protecting not just Mr. Trump’s personal reputation, but his political fortunes.

“I would be your eyes and ears,” Mr. Pecker recalled telling them, as he explained the tabloid practice of “catch and kill,” in which an outlet bought the rights to a story, only to never publish it.