The last time the two New York tough guys were in the same room, one was the American president and the other was his loyal fixer.
Shortly after that encounter, one of the men, Michael D. Cohen, turned on the other, his former boss, Donald J. Trump. In the five years since, Mr. Cohen has gone to prison and testified against Mr. Trump before Congress and a grand jury. Mr. Trump, for his part, has been impeached twice, voted out of office and indicted four times.
Their reunion is now set for a stage that has become familiar to them both: a New York courtroom, where Mr. Cohen will take the stand as soon as Tuesday as a star witness against Mr. Trump in a civil fraud trial.
The testimony will bring to dramatic life a feud that until now has played out on social media and in the pages of Mr. Cohen’s books, one of which he titled “Revenge.” Mr. Trump, who has called Mr. Cohen a “rat” and a liar, scowled at a video clip of him on the trial’s opening day and is expected to attend at least some of his testimony.
The lead-up has had the hype of a heavyweight fight. When Mr. Cohen delayed his testimony, citing a health problem, Mr. Trump claimed he “didn’t have the guts” to testify. Mr. Cohen fired back, posting a mocked-up image of himself and the former president captioned “Let’s get you back to your cell.”
Mr. Cohen’s testimony is expected to begin a more explosive phase of the trial, which stems from a lawsuit brought by the New York attorney general, Letitia James. The suit accused Mr. Trump of inflating the value of his assets by billions of dollars to obtain favorable treatment from banks and insurance companies.
The judge, Arthur F. Engoron, has already ruled that Mr. Trump fraudulently misvalued his properties. The trial will determine whether he has to pay a hefty penalty and whether his conduct violated other laws.
With the central claim resolved, the trial has been a tedious proceeding, punctuated by Mr. Trump’s occasional visits, during which he uses a camera-lined courthouse hallway as a campaign stop. His lawyers have made it clear that they will appeal key rulings by Justice Engoron, who will decide the case: There is no jury. They have argued that valuations are subjective and that others were to blame.
Mr. Cohen’s testimony before Congress in 2019 served as the impetus for Ms. James’s investigation. Mr. Cohen told a committee that the former president’s company manipulated financial statements, including by starting with Mr. Trump’s desired net worth and computing the value of the assets to match it.
He is expected to reprise those comments and fill in details this week. But the testimony’s substance may pale in comparison to the drama of the face-off.
It also will test Mr. Trump’s courtroom decorum on the heels of a $5,000 fine Justice Engoron imposed on the former president for violating a gag order.
The judge acted after Mr. Trump posted a picture on social media of Justice Engoron’s law clerk, Allison Greenfield, with Senator Chuck Schumer, the majority leader. Mr. Trump falsely labeled Ms. Greenfield “Schumer’s girlfriend.” When the Trump campaign failed to remove the post from its website, Justice Engoron levied the fine and threatened to hold Mr. Trump in contempt and even jail him for future violations.
The order applies only to comments about the judge’s staff, but Justice Engoron is also likely to object to threats to Mr. Cohen or other witnesses. Yet threats are the defining feature of Mr. Trump and Mr. Cohen’s relationship.
Mr. Cohen went to work for Mr. Trump in 2007 and embraced the role of attack dog. He once vowed to take a bullet for the man he referred to as “Mr. Trump.”
That all changed in 2018, when Mr. Cohen became the target of a federal investigation into his role in a hush-money payment to a porn star named Stormy Daniels. The payment, which Mr. Cohen made during the 2016 campaign, blocked Ms. Daniels from telling her story of an affair with Mr. Trump years earlier — an affair that Mr. Trump denied ever took place.
In one of their final encounters, Mr. Cohen visited Mr. Trump’s Mar-a-Lago estate in Florida in March 2018. The F.B.I. searched Mr. Cohen’s home and office soon after.
With Mr. Cohen facing legal bills and prison, Mr. Trump began to distance himself from his fixer, who soon lashed out and began to speak with prosecutors. When Mr. Cohen pleaded guilty to campaign finance charges in 2018, he pointed the finger at his former boss, saying in court that he had paid Ms. Daniels at the direction of Mr. Trump.
Federal prosecutors declined to indict Mr. Trump. But Mr. Cohen, who served a little more than a year in prison, repeated his accusations to their state counterparts, who in April announced charges against Mr. Trump related to the deal. Mr. Cohen is expected to be a star witness in that criminal trial, which could start next spring.
In a statement ahead of his testimony on Tuesday, Mr. Cohen said: “It’s been five years since we have seen one another. I look forward to the reunion. I hope Donald does as well.”
