Ex-attorney Michael Cohen took the stand Monday at former President Donald Trump’s ongoing criminal trial, serving as the prosecution’s central witness as he’s expected to testify about the $130,000 payment to adult film star Stormy Daniels at the heart of the trial.

Cohen is expected to be one of the prosecution’s final witnesses at the trial, as prosecutors said Friday they could rest their case as soon as the end of this week.

Cohen, who’s become one of Trump’s most outspoken critics, has criticized Trump throughout the trial. The ex-attorney tweeted in April that he would stop commenting on the trial on X and his “Mea Culpa” podcast after a tweet calling Trump “VonShitzInPantz” drew criticism, but ABC News reports he has continued to speak out against the ex-president on TikTok live streams. Trump is bound by a gag order that bars him from speaking about Cohen and other witnesses—which the ex-president’s lawyers have asked to be overturned in part so Trump can respond to Cohen’s attacks—and Trump attorneys asked Judge Juan Merchan on Friday to impose a gag order on Cohen as well ahead of his testimony. Merchan denied the request but asked prosecutors to tell Cohen “the judge is asking him to refrain from making any more statements,” after legal experts suggested to ABC News that Cohen’s public attacks on Trump could affect the case.

Cohen was long known as Trump’s “fixer”—even at one point saying he would take a bullet for the former president—before flipping on him and becoming one of Trump’s staunchest critics in 2018, when the attorney pleaded guilty to crimes including campaign finance violations connected to his payment to Daniels. He was sentenced to three years in prison for his crimes. Cohen paid Daniels $130,000 right before the 2016 election to cover up her allegations of having an affair with Trump, which Trump then reimbursed through a series of $35,000 payments made throughout 2017. Prosecutors allege those checks—which added up to $420,000, also adding in a separate expense, bonus for Cohen and enough money to cover his taxes on the payment—were falsely labeled as being for legal expenses. Trump has denied those allegations and pleaded not guilty to the charges. Cohen is taking the stand Monday as witnesses throughout the trial have already testified about the ex-attorney—often unfavorably, referring to him as an “aggressive” guy who was often out for his own gain. Prosecutors told the jury in their opening arguments that they would be “upfront” about the fact that Cohen is a flawed witness, citing him pleading guilty to crimes that include lying to Congress, but argued “the evidence will … show why you can credit Michael Cohen's testimony, despite those past mistakes.”