Donald Trump's one-time fixer and the star prosecution witness in the former president's criminal trial testified Monday that he lied and bullied for his former boss whom he said told him to cover up an affair.

Michael Cohen, once Trump's attack dog, gave calm, unemotional evidence against him, occasionally glancing at Trump who sat slouched in his chair at the defendant's table.

Trump is accused of falsifying business records to reimburse Cohen for a hush-money payment to porn star Stormy Daniels before the 2016 election, when her claims of a sexual encounter with Trump could have proved politically fatal.

Prosecutor Susan Hoffinger asked Cohen if he lied and bullied people when working as Trump's personal lawyer.

"Yes... It was what was needed in order to accomplish the task," Cohen told the courtroom, as Trump looked on just six months before the election in which he hopes to retake the White House.

Cohen is key to the prosecution case, but he spent 13 months in jail, and a year and a half under house arrest, after he pleaded guilty in 2018 to lying to Congress and financial crimes.

In the first weeks of the trial, jurors in New York have heard from witnesses that Cohen was a difficult character who cajoled others to get his way, while the defence has painted him as a pathological liar and convicted criminal.

Cohen has long acknowledged arranging the $130,000 "hush money" payment to Daniels in exchange for her silence about the alleged 2006 tryst -- which Cohen described as "catastrophic" and "horrible for the campaign."

Daniels and Trump -- under the respective pseudonyms Peggy Peterson and David Dennison -- were party to a nondisclosure agreement prepared by Cohen that has emerged in court filings.

- 'This is a disaster' -

The payment was revealed by The Wall Street Journal in 2018 and forms the basis for the charges that Trump faces in the trial.

Cohen recalled saying to Trump "we need to take care of it and (Trump) said 'absolutely, take care of it, do it.'"

"'This is a disaster... women will hate me'," Cohen claimed Trump said in response to the situation.

"He told me to work with (tabloid chief David Pecker) and get control over this... we need to just stop this from getting out," Cohen said, detailing a plan to buy the rights to the story -- but to put off paying as long as possible.

"'Just get past the election because if I win it has no relevance, I'll be the president, and if I lose, I don't really care'," Cohen recalled Trump saying.

Cohen told the jury how the Trump campaign would seek to buy unflattering stories, a practice known as "catch and kill," which is what is alleged to have happened with Daniels.

"Because of the nature of the issue (we had) to make sure that it remained private," Cohen said of separate negotiations to buy the story of former Playboy model Karen McDougal who also alleged an affair with Trump.

That deal ultimately did not proceed, although Cohen secretly recorded a conversation with Trump about purchasing such stories.

During nearly eight hours over two days last week, Daniels walked the jury through the one-night stand she said she had with Trump at a celebrity golf tournament, and then the financial settlement.

Trump has denied having sex with Daniels, and his lawyers last week asked Judge Juan Merchan for a mistrial on the grounds her graphic testimony was prejudicial in what is essentially a financial records and election-related case.

Trump, 77, who is on trial while also campaigning to avenge his 2020 loss to President Joe Biden, could still stand in the November election and be sworn in as president if he was to be convicted and even jailed.

Trump's son Eric, who was joined in court by Senator J.D. Vance, a contender for Trump's vice presidential pick, tweeted that he had "never seen anything more rehearsed" than Cohen's testimony.

In addition to the New York case, Trump has been indicted in Washington and Georgia on charges of conspiring to overturn the results of the 2020 election.

He has also been charged in Florida with allegedly mishandling classified documents after leaving the White House but that case has been postponed indefinitely.