New York, New York - Donald Trump's one-time fixer and the star prosecution witness in the ex-president's criminal trial Michael Cohen testified Monday that he lied and bullied to help his former boss hide "catastrophic" revelations of a tryst with an adult film actress.

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 $130,000 hush-money payment to adult film star Stormy Daniels before the 2016 election when her claims of a sexual encounter with Trump could have doomed his campaign.

"I spoke to Mr Trump and we expressed to him that I was going to front the money for it, for which he was appreciative," Cohen said of a conversation he and Trump's finance chief Allen Weisselberg had with the mogul.

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"Good, good," Trump said according to Cohen.

"He stated to me, 'Don't worry, you'll get the money back,'" he told the court.

Cohen is critical to the prosecution's case, but his credibility will be put to the test. He spent 13 months in jail and another year and a half under house arrest after pleading guilty in 2018 to lying to Congress and committing 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 defense 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 2006 alleged sexual liaison – which Cohen described as "catastrophic" and "horrible for the campaign."

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 less than six months before the election in which he hopes to retake the White House.

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.

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.

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"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.

During nearly eight hours over two days last week, Daniels walked jurors through the encounter 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.