WASHINGTON — Harry Reid lived for the Senate floor. He also lived on it.

When Mr. Reid, a senator from Nevada, became the Democratic whip in 1999, he eagerly assumed most of the day-to-day floor responsibilities for Tom Daschle, then the majority leader, essentially taking up permanent residence in the chamber and becoming as much of a fixture as the desks.

“He virtually wore the carpet down between the floor and my office,” said Mr. Daschle, noting that Mr. Reid would traipse over a dozen or more times a day to consult, enough that Mr. Reid’s wife, Landra, would wait for her husband in Mr. Daschle’s lobby, knowing she could eventually catch him there.

On Wednesday, Mr. Reid will lie in state in the Capitol Rotunda, just down the hall from the chamber where he ensconced himself and then presided as majority leader from 2007 to 2015. His tenure included a three-year period from 2008 to 2011 when the Senate produced an extraordinary burst of legislation, including the Affordable Care Act, a flurry of policymaking that seems almost unthinkable today with the chamber mired in dysfunction and stalemate.

Lying in state in the Rotunda is a rare honor, one that he would have never imagined for himself when he happily took on the floor drudgery as Democratic whip, the second-ranking position in his caucus. That work was ideally suited to an inside player like Mr. Reid, a law student who moonlighted as a Capitol Police officer and became a leading congressional figure.

But his choice to devote himself to internal machinations turned out to be a shrewd move in an unlikely political career filled with them. His devotion to the minutiae of the Senate schooled him in the arcane rules of the institution, and it also gave him a chance to learn what made his colleagues tick, what they needed and wanted, and how he could satisfy those demands to secure their votes when the moment of truth arose.

It was knowledge that came in handy in November 2004 when Mr. Daschle lost his re-election bid in South Dakota and President George W. Bush defeated his Democratic challenger, John Kerry, to win a second term. Some Democrats wondered if the party needed a more charismatic leader than Mr. Reid, a soft-spoken and quirky former boxer from the tiny town of Searchlight, Nev., who was occasionally awkward and not exactly a mesmerizing media presence.

But by the time the idea even occurred to others, Mr. Reid had already used his well-honed skills to nail down the votes for the job.

“I thought about it for an hour,” said Christopher J. Dodd, then a Democratic senator from Connecticut, who said he quickly realized that challenging Mr. Reid would be a “fool’s errand.”

“It was quite obvious that Harry locked this thing up,” said Mr. Dodd, acknowledging that Mr. Reid had carefully built up his base of supporters.

“He was on the floor every day, all day, and he knew every member, what their difficulties were and the things they liked,” said Mr. Dodd, an ally of Mr. Reid who said he had grown even closer to his former colleague in their post-Senate lives. “It was exactly the kind of work you needed to perform as majority leader.”

Raised in poverty with none of the advantages that would seem to be prerequisites for powerful national office, Mr. Reid was combative but also compassionate, an attribute that contributed to his eventual zeal for laws to protect undocumented immigrants brought to the United States as children.

He could pound an adversary but, in recognition that someone’s vote would be needed in the future, he left room for working together again. He knew how to cajole and bargain to win votes, but he could be ruthless as well.

“Harry Reid was a very complex person,” said Senator Susan Collins, Republican of Maine, a frequent target for criticism from Mr. Reid but also someone who provided him with critical votes for the economic stimulus bill and the banking overhaul during the Obama era, among other measures. “He could be incredibly kind and caring, and he could be utterly relentless. It was a real mixture, but he did care about the Senate and the senators.”

Ms. Collins, one of the Senate Republicans who attended Mr. Reid’s memorial service on Saturday in Las Vegas, has held onto personal notes from her occasional adversary, including one commending her “tenacious” negotiating skills and signed “your new friend forever.”

Mr. Reid was willing to sacrifice personally in pursuit of what he saw as the greater political goal. With the Senate divided 50-to-50 after the 2000 elections, Democrats were courting two Republican senators, John McCain of Arizona and James M. Jeffords of Vermont, to become independents and hand Democrats control.

Mr. McCain wanted to become chairman of the Commerce Committee in return, but Senator Ernest F. Hollings, Democrat of South Carolina, was having none of it and would not relinquish his post.

Mr. Reid, in contrast, gave up his chairmanship of the Environment and Public Works Committee — almost unthinkable to most senators — to woo Mr. Jeffords. The Vermont senator ended up switching parties, upending the Senate’s balance of power in favor of Mr. Reid and the Democrats.

“That was the deciding factor,” Mr. Daschle said. “Harry played the most amazing role.”

Mr. Reid’s pugnacious nature could land him in trouble. He made a litany of remarks that could be considered politically incorrect at best and offensive at worst, often requiring rapid cleanup by staff members who sometimes cringed when he spoke with the news media.

“Reid was really a blunt, plain-spoken individual who would say what was on his mind,” said Jim Manley, Mr. Reid’s longtime Senate spokesman and communications chief, who said he found the senator’s candor and willingness to stir things up refreshing.

“But it was not without political peril,” he said. “And as his spokesman, it gave me lots of gray hair.”

Mr. Reid was able to maintain friendships across the aisle despite occasional rifts. He was willing to mend fences with the equally fiery Mr. McCain, for instance, after several clashes, and he remained close with Senator Richard C. Shelby of Alabama, who switched from Democrat to Republican in 1994.

The friendship between Mr. Reid and Mr. Shelby — two lawmakers who enjoyed using their congressional expertise to get what they wanted — dated from their early days as neighboring House members in the Longworth House Office Building.

“We were helpful to each other where we could be,” Mr. Shelby said in his typically understated fashion. “Harry was a practical man and very underestimated as far as climbing that political ladder inside the Senate.”

“He had a lot of integrity,” said Mr. Shelby, who also attended Mr. Reid’s memorial service. “He was steadfast in his beliefs and he was honorable and was loyal to his friends.”

Even when he returned to Nevada full time after not running for re-election in 2016, and long after he was diagnosed with pancreatic cancer, Mr. Reid continued to play power politics until the end.

When Rahm Emanuel, the former White House chief of staff and congressman, was working in September to round up votes in support of his nomination to be ambassador to Japan, he reached out to Mr. Reid, with whom he had worked closely during the first two years of the Obama administration. Mr. Emanuel was looking for help in connecting with Nevada’s two Democratic senators, both protégés of Mr. Reid.

He did not have to wait long. Less than 10 minutes after sending his inquiry, he received his answer via text: “both are for emanuel going to Japan! H Reid.”