But seven years later, Cohen will play the role of the prosecution's star witness in the first criminal trial of a former U.S. president.

Trump faces 34 counts of falsifying business records to hide a $130,000 hush-money payment to porn star Stormy Daniels to keep her quiet about what she says was a 2006 sexual affair.

The former president has pleaded not guilty and denied any relationship with Daniels.

But in court, Trump will face the testimony of someone who was once among his closest confidantes.

Luc Cohen - no relation - is covering Trump's New York trial for Reuters.

"So Michael Cohen, for years until Trump became president, was his loyal deputy, right. He was his lawyer, his quote unquote, "fixer". So basically, you know when problems arose for Trump while he was working in in the real estate industry, Michael Cohen basically took care of them. He was known for being very loyal to Trump."

Cohen pleaded guilty to breaking federal campaign finance laws over his role in the alleged hush money scheme. He served time in jail for that and other crimes, including cheating on his personal taxes, and lying under oath to Congress.

MICHAEL COHEN: "Mr. Trump is a con man. He asked me to pay off an adult film star with whom he had an affair and to lie about it to his wife, which I did. And lying to the first lady is one of my biggest regrets, because she is a kind, good person and I respect her greatly, and she did not deserve that."

Cohen turned on Trump amid that investigation and testified about it in 2019.

LUC COHEN: "He ended up pleading guilty and and and breaking with Trump right so and and you know he did pay a heavy price he was sentenced to three years in prison served about a year of that before he was released. To to home confinement"

Now the former fixer stands ready to help Manhattan District Attorney Alvin Bragg criminally prosecute Trump. This is what Cohen told reporters before speaking to a grand jury last year:

JOURNALIST ASKING (OFF CAMERA): "What will be the result of your testimony today? Can you imagine Mr. Trump being arrested? Is that what you want?"

MICHAEL COHEN: "No. Again, you've heard me say this many times. This is not revenge. What this is about accountability. I don't want to see anyone, including Donald Trump, indicted, prosecuted, convicted, incarcerated, simply because I fundamentally disagree with them. This is all about accountability. He needs to be held accountable for his dirty deeds."

Trump's attorneys are expected to take aim at Cohen's credibility.

LUC COHEN: "Michael Cohen has pleaded guilty to to perjury. He's admitted to lying to Congress about a a real estate project that the Trump Organization had in Russia. Now Michael Cohen says he, he, he Michael Cohen admits to lying. He says he did. He did so to protect Trump.

But he is still, you know, an admitted perjurer right pleaded guilty to perjury. That is, you know, going to be a liability for him on on the witness stand.

Trump made this point ahead of Cohen's testimony in a civil fraud case against the former president.

"He is a proven liar, as you know, he's a felon. He's served a lot of time for lying, and we're just going to go and see and I think you'll see that for yourself."

Trump has called the case against him a politically-motivated witch hunt meant to harm his candidacy as he mounts a third run for the White House.

The trial is expected to run through May.