Michael Cohen could make or break Trump as he takes stand on Monday

The prosecution's star witness in Donald Trump's Manhattan criminal trial also could be its biggest liability.

Michael Cohen, Mr Trump's former personal attorney, is expected to take the stand on Monday to testify against the man he once reverently referred to as "The Boss."

Cohen made the hush money payment at the centre of the case, a sum which the prosecution alleges was fraudulently reimbursed by Mr Trump. He could provide essential testimony for prosecutors.

But outside the courtroom, on podcasts, television and social media, Cohen has not exactly helped their case.

He has mocked Mr Trump on X, calling him "sleepy Donald" and has used a profane nickname referring to the former president. Cohen also has posted memes that appear to show Mr Trump in prison-uniform orange, and joked on TikTok about Mr Trump being incarcerated.

"Trump 2024?" he said during one TikTok stream, reported by ABC News. "More like Trump 20-24 years."

Cohen's actions went too far for Justice Juan Merchan, who on Friday warned prosecutors to tell Cohen to stop commenting on the case.

One of his lawyers, Jeffrey K Levine, told the BBC, "I have no doubt that Michael will cooperate with whatever the court's asking him to do. Or not to do."

Cohen's rogue behaviour, coupled with his very real criminal record, has opened the door for Mr Trump's defence to cast doubt on this crucial player.

When Trump ran for president in 2016, Cohen was often by his side

"He's a real headache," said Lance Fletcher, a former Manhattan prosecutor who now practises criminal defence. "He's doing everything as a prosecutor you don't want your witness to do. He's got all sorts of credibility problems."

In 2018, Cohen pleaded guilty to eight felony counts including tax offences, fraud and campaign finance violations. The latter charge stemmed from the very same hush money payment at the heart of Mr Trump's case.

He also pleaded guilty to lying to Congress - a fact Mr Trump's attorneys have brought up in this trial and during a separate civil case.

Mr Trump is charged with 34 counts of falsifying business records for allegedly reimbursing Cohen for a $130,000 hush money payment he made to adult film star Stormy Daniels, then trying to disguise records of the payment as legal expenses.

Ms. Daniels has alleged that she had sex with Donald Trump in 2006. Mr Trump has pleaded not guilty to the charges and denied any sexual encounter with Ms Daniels.

Todd Blanche, Mr Trump's lead attorney, tried to undercut Cohen as a witness in his opening statement. He said Cohen remained "obsessed" with Trump "to this day," and that he "rants and raves" about the former president.

BBC News reporters are in the Manhattan courtroom covering the historic first criminal trial of a former US president. You'll find their updates and analysis on the BBC news website and app, and across TV, radio and podcasts.

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"I submit to you that he cannot be trusted," Mr Blanche said.

Mr Fletcher agrees with the defence's strategy. "You almost want to make the case about him," he said.

But the prosecution likely plans to use Cohen to link the alleged cover-up of the hush money payment directly to the former president. Prosecutors are almost certain to ask him whether he discussed the matter with Mr Trump, or took direction from him, as they try to establish intent.

Mr Blanche and his colleagues could bring up Cohen's previous comments while he's on the stand, and call into question his motivation for providing damaging testimony against Mr Trump. His attorneys already have alleged Cohen is profiting off Mr Trump's legal problems through his podcast, TikTok streams, and books.

If Cohen testifies, "maybe you can get him upset, maybe he starts yelling and screaming and coming apart at you," Mr Fletcher said.

It's a tactic that worked in Mr Trump's civil business fraud trial last year. When Cohen testified, he got into a brief shouting match with one of Mr Trump's attorneys after she repeatedly called his credibility into doubt.

Rolling the dice

During her time as a prosecutor, former New York State Acting Supreme Court Justice Diane Kiesel used to have a saying: "I don't go to central casting for my witnesses."

In other words, prosecutors do not always get the perfect witness to prove their case - some may provide essential testimony, but are flawed. Manhattan District Attorney Alvin Bragg is in a similar position with Cohen, she said.

"Who do you think is going to be making hush money payments to a porn star?" former Judge Kiesel said. "Some upstanding member of the bar... who everyone looks up to as a paragon of legal ethics?"

She said Mr Bragg's team must "go through every single thing that Michael Cohen has to say" on the stand and offer corroborating evidence to convince the jury.

But prosecutors cannot make Cohen stop talking outside court.

"When I was a prosecutor, I [occasionally] wished I could give witnesses a gag order, but you can't," Mr Fletcher said. "They have the same rights and powers as regular citizens."

Prosecutors acknowledged as much on Friday, after Mr Blanche argued the former president should be allowed to respond to Cohen despite the gag order barring him from speaking about witnesses.

Assistant District Attorney Joshua Steinglass responded that they had asked Cohen "repeatedly" not comment on the case, but his office had "no remedy."

"The judge is asking him to refrain" from making statements, Justice Merchan said firmly. "That comes from the bench."

Prosecutors already have acknowledged Cohen's flaws to jurors.

During opening statements, assistant district attorney Michael Colangelo told the jury: "You will need to keep an open mind" about Cohen, and "keep in mind all the evidence that corroborates Michael Cohen's testimony".

Prosecutors have introduced a paper trail of texts, emails, phone calls, bank records and legal contacts Cohen left as he brokered payouts to Ms Daniels and another woman, Playboy model Karen McDougal, in exchange for their silence.

They have called on multiple corroborating witnesses, like former tabloid publisher David Pecker and Ms Daniel's former attorney Keith Davidson, to shed light on events involving Cohen.

The district attorney's office did not return a request for comment for this report.

On 24 April, Cohen announced - likely to the temporary relief of prosecutors - that he would impose a gag order on himself.

"Despite not being the gagged defendant, out of respect for Judge Merchan and the prosecutors, I will cease posting anything about Donald on my X (formerly Twitter) account or on the Mea Culpa Podcast until after my trial testimony," Mr Cohen posted. "See you all in a month (or more).

But in its complaint last week, the defence cited social media jabs at Mr Trump that Cohen had posted more recently.

The DA can ''read him the riot act all they want,'' said former Judge Kiesel. "But if he's not gonna listen, he's not gonna listen."