The first criminal trial against a US president reaches a climax when Michael Cohen, a key witness, appears on the scene.

Donald Trump’s former personal henchman and attorney Michael Cohen will testify in Trump’s hush-money trial this week, AP reports. Cohen is considered the prosecution’s most important witness in Manhattan. His testimony could be decisive for the outcome of the first criminal trial against a US president.

Cohen is expected to reveal details about his role in arranging hush money payments during Trump’s first presidential campaign. These include payments to porn actress Stormy Daniels, who testified in court last week. The $130,000 she received in 2016 was supposedly intended to prevent her from speaking publicly about a sexual encounter she claims she had with Trump in a hotel suite a decade earlier.

Testimony from a witness with detailed insight into Donald Trump’s actions could increase legal risk for the potential Republican presidential candidate if the court finds the witness credible, the Associated Press reports. But the fact that prosecutors are relying on a witness with a shady past could also prove to be an advantage for Trump, who portrays the case as the product of a tainted justice system. Cohen pleaded guilty to lying to Congress.

Cohen’s role as a key prosecution witness continues to highlight the breakdown of a relationship that was once so close that Cohen said he would “take a bullet for Trump.” After the FBI raid on Cohen in 2018, Trump praised him on social media as a “fine person with a wonderful family” and incorrectly predicted that Cohen would not “turn around.”

Cohen later admitted lying to Congress about a Moscow real estate project he pursued on Trump’s behalf during the height of the 2016 Republican campaign. He claimed he lied to agree with Trump’s “political message,” according to AP.