Pecker said the decision to bury the stories followed a 2015 meeting at which he told Trump that the Enquirer would publish favourable stories about the billionaire candidate and keep an eye out for people selling stories that might hurt him. He said he told an editor to keep the arrangement secret.

Pecker said the Enquirer paid former Playboy model Karen McDougal for her story of a sexual relationship with Trump in 2006 and 2007. He said he bought the story after Trump refused to do so himself.

"He said that anytime you do anything like this it always gets out,” Pecker said.

The Enquirer's parent company, American Media, said in 2018 it paid $US150,000 ($A231,000) for the story. Trump has denied having an affair with McDougal.

The tabloid also paid $US30,000 ($A46,000) for a story peddled by Trump Tower doorman, Dino Sajudin, who claimed Trump fathered a child with a maid who worked for him. The story turned out not to be true, Pecker said.

Both payments far exceeded the amounts the paper typically paid for stories, he said.

“I made the decision to buy the story because of the potential embarrassment it would have to the campaign and Mr. Trump,” Pecker said.

He is expected to testify further when the trial resumes on Thursday.

Prosecutors say Pecker's actions helped Trump deceive voters in the 2016 election by burying stories of alleged extramarital affairs at a time when he already faced multiple accusations of sexual misbehaviour.

They have charged Trump with criminally falsifying business records to cover up a $US130,000 ($A200,000) payment to buy the silence of porn star Stormy Daniels, who says they had a sexual encounter 10 years earlier.

Trump has pleaded not guilty and denies having an encounter with Daniels. His lawyers argue that Trump did not commit any crimes and only acted to protect his reputation.

The case may be the only one of Trump's four criminal prosecutions to go to trial before the Republican's November 5 election rematch with Democratic President Joe Biden.

A guilty verdict would not bar Trump from taking office but could hurt his candidacy.

Pecker's testimony came after a hearing to consider prosecutors' request to fine Trump $US10,000 ($A15,000) for violating a gag order prohibiting him from criticising witnesses, court officials and their relatives.

After the session, Trump repeated his claim that the gag order violated his constitutional free speech rights.

"This is a kangaroo court and the judge should recuse himself!" Trump wrote on his Truth Social platform.

New York prosecutor Christopher Conroy said Trump has run afoul of the order, pointing to an April 10 Truth Social post that called Daniels and Trump's former lawyer Michael Cohen "sleazebags." Both are expected to testify in the first criminal trial of a former US president.

Conroy said other posts led to media coverage that prompted a juror last week to withdraw over privacy concerns.

"He knows what he's not allowed to do and he does it anyway," Conroy said of Trump. "His disobedience of the order is wilful. It's intentional."

The $US10,000 ($A15,000) fine sought by Conroy would be a relatively small penalty for Trump, who has posted $US266.6 million ($A411 million) in bonds as he appeals civil judgments in two other cases.

Conroy said he was not at this point asking Merchan to send Trump to jail for up to 30 days, as New York law allows.

"The defendant seems to be angling for that," Conroy said.

Blanche said Trump's posts were responses to political attacks by Cohen and not related to his former lawyer's expected testimony.

"He's allowed to respond to political attacks," Blanche said.