The Democrats’ lead impeachment hearing lawyer made his bones as a prosecutor by sending mobsters, stock swindlers and a multimillion-dollar insider trader to prison, cases in which colleagues said he mixed brains and “swagger” to win convictions.

Daniel S. Goldman spent a decade as an assistant U.S. attorney in Manhattan, a jurisdiction known for its tough, high-profile cases. He left that job in 2017 to become a television legal analyst but now holds a weightier role questioning witnesses called to testify about President Trump’s effort to persuade Ukraine to investigate a political rival.

At the public hearings before the House Intelligence Committee due to begin Wednesday, Goldman is slotted to question each witness for 45 minutes, followed by five-minute question sessions for each lawmaker. Stephen R. Castor, general counsel for the House Oversight Committee, will be the Republicans’ point man.

The format is a significant departure from routine congressional hearings, where lawmakers have the spotlight and seldom cede the microphone and live television coverage to a staffer. By assigning a big chunk of the questioning to a committee lawyer, and in Goldman’s case, an accomplished former prosecutor, party leaders are tacitly acknowledging just how serious the stakes are.

Friends and colleagues said Goldman has had tremendous training for the bright lights of the national stage.

In 2017, when then-Assistant U.S. Attorney Brooke Cucinella was preparing for the trial of Billy Walters, a professional gambler who made millions on insider stock trades, she knew it would make big headlines, in part because the case involved champion golfer Phil Mickelson.

Cucinella said she requested Goldman to be on the trial team “because he has a bit of a swagger as a trial lawyer, and it’s a confidence that serves him well. In a courtroom, he’s in­cred­ibly effective.”

Walters, one of the most successful sports gamblers in Las Vegas history, rolled the dice against Cucinella and Goldman and lost. He was convicted on all counts and sentenced to five years in prison. But after the verdict, on the night prosecutors took out the FBI case agent for thank-you drinks, Goldman received a devastating phone call — his brother had died in a plane crash.

It was not the first time his family had suffered a terrible loss. Goldman’s father, a federal prosecutor in Washington, died when Goldman was a boy.

“He’s an incredible family person,” Cucinella said. “I don’t quite know how to say this, but I think he’s humble enough to get input from other people and will take into account what really matters to people.”

At points in his life, Goldman has shown his sensitive side. As a new legal analyst for NBC, he appeared last year on the ­now-defunct “Megyn Kelly Today” show to talk about wedding engagement stories.

Goldman described how he proposed to his wife — a funny story featuring a ring, a fortune cookie and a foot of fresh snow. At the end of the story, his fellow panelists laughed, and Kelly told him, “You’re a romantic at heart.”

That video clip made the rounds among his former colleagues in the prosecutor’s office, who mocked him for months over it, those close to him said. But people who have worked alongside Goldman describe him as a meticulous lawyer who can absorb volumes of information and distill it in a meaningful way for a jury.

“He was one of the go-to trial guys, and there’s no case that’s too complex for him. He’s a great prosecutor and he’s got a very powerful presence,” said Preet Bharara, the former U.S. attorney for the Southern District of New York who was Goldman’s boss for years. “To the extent the public will be watching and looking for a credible questioner, I think they will be very impressed with Dan Goldman.”

Bharara said the impeachment hearings will be challenging for even a skilled trial lawyer, because of the proceeding’s unusual nature.

Impeachment “is fully, 100 percent a legal event, in that it’s a constitutional enterprise right from the start,” he said. “At the same time, the nature of the constitutional proceeding is a dynamic that includes two political branches, so it’s political in that sense, but not political in the pejorative sense. That’s how the Founders wanted this to happen.”

Bharara noted an incident last month when one of the closed-door impeachment hearings was stormed by angry Republican lawmakers, some of whom tweeted from inside a room for sharing classified information.

“You have to be calm, and strong, and roll with the punches, and I think Dan has always had his wits about him,” Bharara said.

Eight years ago, Goldman prosecuted a number of Genovese mobsters, including a hit man named Fotios “Freddy” Geas. The hit man was sentenced to life in prison for his crimes, but he would become an even bigger name in the criminal world as an inmate. Last year, Geas was identified by law enforcement officials as the prime suspect in the slaying of notorious Boston mob boss James “Whitey” Bulger — a prison killing for which no one has been charged.

After the mobster cases, Goldman prosecuted a sprawling auto insurance fraud case involving Russian organized crime figures. Dozens of suspects were charged in the scheme to rip off insurance companies for more than $100 million by submitting false bills for fake injuries. Officials said the key players in the group were all named Mike, so the gang used nicknames: Russian Mike, Skinny Mike, Fat Mike and Mike B.

Goldman also won convictions in that case.

Days after the House Intelligence Committee chairman, Rep. Adam B. Schiff (D-Calif.), hired Goldman in March, he brought in another former prosecutor from the Southern District of New York, Daniel Noble, to work alongside Goldman.

Each brings a wealth of prosecutorial experience to the impeachment inquiry, but congressional hearings are far different from a courtroom in critical respects. There are no federal rules of evidence to follow, nor a judge to make snap decisions on the evidence or arguments. Questions are limited by the clock.

But the impeachment inquiry to date has been strikingly similar in one respect to the rituals of criminal prosecution: Many of the key witnesses in impeachment have testified behind closed doors, similar to the way in which prosecutors will use a grand jury to lock in witness statements before outside influence or fuzzy memories weaken the evidence.

The committee, with Goldman doing a great deal of the questioning, has nailed down the accounts of a number of key witnesses, though others have so far resisted subpoenas for their testimony.

Goldman’s task will be to present a compelling portrait of all that Democrats have uncovered in recent weeks, and to convince the public that the president’s actions merit impeachment.

“This is probably the toughest jury he or any of them will ever face,” said Goldman’s former boss, Mimi Rocah. “When people are in courtrooms deciding a criminal trial, they really are able to put aside personal and political beliefs. Here, that’s harder to do, because people are at home watching television, reading it all through a certain lens.”

Goldman “has done big trials, and big trials are about distilling a mountain of information down to what’s important. He is very good at that,” Rocah said. “Dan Goldman is about to become a household name.”