For weeks, the spectre of Cohen's testimony has hovered over the trial, his name repeatedly invoked as prosecutors walked jurors down a trail of wheeling and dealing in bank statements, text messages, witness testimony, emails and recordings of crucial conversations.

Sixteen days into the trial, he finally appeared on the witness stand, the most significant day of testimony in the first-ever criminal trial of a former US president.

What he had to say about events and conversations from eight years ago could come back to haunt Mr Trump as he seeks to return to the White House.

Cohen, who served time in prison for crimes related to the payment, is one of the few witnesses who could help prosecutors establish that Mr Trump knew about the alleged reimbursement scheme. He could also testify about reasons why Mr Trump allegedly committed fraud. Prosecutors have said it was to interfere in the 2016 election.

On Monday Cohen also testified that Mr Trump told him to "be prepared" for negative publicity after announcing his 2016 presidential bid,

"There's going to be a lot of women coming forward," he allegedly said.

He said Mr Trump instructed him to "handle" several hush-money payments, as well as liaise with tabloid media to thwart negative stories that could threaten his political goals.

He recalled Mr Trump being "really angry" for what he perceived as a failure to keep Ms Daniels from shopping her story to the media.

He said Mr Trump told him to "just take care of it", calling the story a potential disaster for his presidential campaign.

"Women will hate me," he told Cohen, according to Monday's testimony.

Cohen also testified that he worked with the National Enquirer tabloid to buy and bury a story about an alleged affair between Mr Trump and Playboy model Karen McDougal.

Cohen said he was present for a call in which Mr Trump asked National Enquirer publisher David Pecker about a payment to Ms McDougal.

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"Pecker said 'we have this under control, we'll take care of this,'" Cohen testified.

Speaking to reporters later outside the court, Mr Trump complained that the case was politically motivated and an attempt to derail his presidential campaign.

Though Mr Trump is prohibited from publicly commenting on witness testimony, Republican senators Tommy Tuberville of Alabama and JD Vance of Ohio appeared in a show of support, assailing Cohen and defending the former president.

"The thing that the president is prevented from saying, which is a disgrace, is that every single person involved in this prosecution is practically a Democratic political operative," Mr Vance said.

Norm Eisen, an attorney who interviewed Cohen in 2019 while assisting the House of Representatives, told the BBC that Cohen's testimony so far had boosted the prosecution's case.

"Given disparaging references others have made to Cohen, I am sure the jury is surprised by the sober, well-spoken, candid individual they are meeting," he said.

"Of course, final judgment must wait until after cross-examination and a verdict, but I think he is doing well so far," he added.

In 2018, Cohen pleaded guilty to eight felony counts, including tax offences, fraud and campaign finance violations - the latter of which stemmed from the very same payment in question at Mr Trump's trial.

He also pleaded guilty to lying to Congress.

Mr Trump's lawyers have sought to distance Mr Trump from the payment.

In earlier testimony, they asked Mr Trump's former communications aide, Hope Hicks, if Cohen would sometimes go "rogue."