By: Emily Cochrane of The New York Times

WASHINGTON — On New Year’s Day in 1999, Representative Jim Sensenbrenner, Republican of Wisconsin, sat on the floor of his Capitol Hill office, surrounded by piles of documents and legal notes, drafting his opening argument in President Bill Clinton’s impeachment trial in the Senate.

With the sound of the University of Wisconsin Badgers facing off against the U.C.L.A. Bruins in the Rose Bowl blaring from a television in the background, Mr. Sensenbrenner readied his case that the president should be removed from office for lying about a sexual affair with a White House intern.

In the coming days, Speaker Nancy Pelosi, Democrat of California, is expected to select four to 10 members of the House of Representatives for a similar assignment, making the case to the Senate for why President Trump deserves to be ousted for pressuring Ukraine to investigate his political rivals. But unlike Mr. Sensenbrenner or the dozen other prosecutors who made the case against Mr. Clinton 21 years ago, the new prosecutors will not have had the benefit of a two-week holiday break to prepare their arguments or hone their strategies.

On Friday, after a weekslong impasse, Ms. Pelosi alerted lawmakers that she would move next week to transmit the articles of impeachment to the Senate, prompting the start of a Senate trial as early as Wednesday.

Ms. Pelosi’s decision to withhold the articles of impeachment in an unsuccessful effort to extract assurances from Senate leadership about the terms of the trial has delayed the appointment of the so-called impeachment managers, raising the stakes and compressing the timetable of their already challenging task.

It is a job that veterans say is fraught with legal complexity, political pressure and historic significance.

“I really don’t want to give them any advice,” Mr. Sensenbrenner said in an interview. “But I guess I can say is that this is going to be a lot more work than you think.”

“The American people,” he added, “are going to be watching.”

The pivotal role of the managers is one reason that Ms. Pelosi has waited to send the charges to the Senate. Senator Mitch McConnell, Republican of Kentucky and the majority leader, said this week that he had the votes to move forward with an impeachment trial without committing to calling witnesses or hearing new evidence. Without knowing whether there will be witnesses to question or new complex documents to digest, the speaker cannot decide what kind of lawmakers are best suited to the task.

People close to Ms. Pelosi say it is all but certain that one of the managers will be Representative Adam B. Schiff of California, a former federal prosecutor who oversees the Intelligence Committee and led the investigation into Mr. Trump’s dealings with Ukraine. Representative Jerrold Nadler of New York, the chairman of the Judiciary Committee that approved the two articles of impeachment against the president that the House passed last month, is also widely expected to be a leader of the group. But for now, final decisions on the rest of the team remain unresolved.

Times have changed considerably since 1998, when the Republican-led House sent 13 white men to the Senate to serve as impeachment managers. Given the diversity of today’s rank-and-file Democrats, Ms. Pelosi is likely to select a group to prosecute Mr. Trump that includes women and members of color.

Mr. Sensenbrenner had previously served as an impeachment manager in the case of Walter Louis Nixon Jr., a federal judge who was impeached and removed from the bench for lying to federal grand juries. In his preparation for the Clinton trial, Mr. Sensenbrenner recalled being given some wry advice from the House Judiciary Committee chairman at the time, Henry J. Hyde, Republican of Illinois, who instructed him to keep his opening statement shorter than the two-and-a-half-day speech that kicked off President Andrew Johnson’s impeachment trial in 1868.

Another impeachment manager, James E. Rogan, a Republican from California, was chosen despite having just joined the House in 1997, in part because he had previously been a prosecutor, which had helped land him a seat on the Judiciary Committee.

Mr. Rogan was defeated by Mr. Schiff in 2000 and is now a state trial court judge in California. A framed poster that hangs above the door to his judicial chambers in Orange County reminds him daily of the vitriol he attracted as one of Mr. Clinton’s main antagonists. “People unite! DENOUNCE ROGAN!!!” it reads.

“I’ve been through one of these before,” Representative Steve Chabot, Republican of Ohio, one of the 13 Clinton impeachment managers, said at a hearing last month. “And they’re ugly. So I have a lot of sympathy for the House managers that are going to be picked.”

In the years since Mr. Clinton’s trial, the job of the managers has only grown more complex. The debate is also more starkly partisan than it was then, with almost no defections from either side. Social media now allows for running commentary online, including by the president himself. During the House’s impeachment inquiry, Mr. Trump used Twitter to disparage witnesses as they were testifying against him.

“I guess what the president was doing is doing his own cross-examination on Twitter,” Mr. Sensenbrenner observed.

Nearly all of the 13 managers from Mr. Clinton’s trial are still alive and three remain in Congress: Mr. Sensenbrenner and Mr. Chabot both still sit on the Judiciary Committee and voted last month against impeaching Mr. Trump. The third is Lindsey Graham, Republican of South Carolina, who is now a senator and the chairman of that chamber’s Judiciary Committee. He will serve as one of 100 jurors in the Senate trial.

“To see Jim Sensenbrenner and Steve Chabot up there, and Lindsey Graham up on the other side, it is sort of odd,” said former Representative Bob Barr, Republican of Georgia, another of the Clinton impeachment managers. “Here we are 20 years later, and we’re doing the exact same thing again.”

Some have revisited their experience, publicly and privately, as the country readies for another impeachment trial. Mr. Sensenbrenner consulted with Republican leaders in the House before the vote on impeachment articles, and Mr. Graham has described his experience to Republican lawmakers on both sides of the Capitol about his experience. Mr. Rogan said that one Democrat on the Judiciary Committee, whom he declined to name, had reached out to him for advice. Others have been less candid about their thoughts on the current impeachment proceedings. Charles T. Canady, who is now chief justice of the Florida Supreme Court, has declined all interview requests on the subject.

“They need to understand the seriousness of what they’re going into,” said Bill McCollum, a former representative from Florida who was one of the Clinton impeachment managers. “They’re going to be doing something very unique — very few people have done what they’re about to do.”

The job comes with political risks. Mr. Rogan lost his seat to Mr. Schiff in part because Democrats targeted him for his role in the impeachment proceedings. Democrats argue that current Republicans in Congress will suffer electoral retribution for their united defense of Mr. Trump.

Mr. Sensenbrenner, who is not running for re-election after more than 40 years in the House, acknowledged that retirement absolved him of any concerns about being punished by voters as a consequence of his full-throated defense of Mr. Trump.

“I really can avoid people coming up to me back home, saying, ‘I’m never going to vote for you again, because you did this,’” he said.

Mr. Sensenbrenner’s perspective on the charges against Mr. Trump — he calls them “phony” — are the opposite of how he viewed those made against Mr. Clinton 21 years ago.

But the experience still feels familiar, he said.

“I’m going through the déjà vu period of my life,” he said as he left the House floor before the impeachment votes. “I never thought I would have to do this again.”