WASHINGTON » It was, Sen. Harry Reid of Nevada ebulliently proclaimed, a "happy day for me" as he stood with Wayne LaPierre, the head of the National Rifle Association, in 2010 at a new shooting range in Las Vegas made possible by federal money secured by Reid. "People who criticize this probably would criticize baseball," Reid said before firing off a few rounds.

These days, Reid, the Senate majority leader, is far more likely to meet with Mayor Michael R. Bloomberg of New York, an outspoken advocate of stricter gun control, than with LaPierre as he prepares to bring the most expansive package of gun safety legislation in a decade to the Senate floor over the next few weeks.

Reid’s evolution from a hard core proponent of gun rights to the shepherd of legislation that would expand background checks, among other gun control measures, emerges from a complex web of political calculations that have come to define his leadership style over the last decade.

How tenacious Reid is willing to be — and whether he will extract votes one by one as he has for other big pieces of legislation — may well determine the fate of the measures.

Reid declined to be interviewed but answered questions by email.

"The families of Newtown and Aurora and the victims of gun violence everywhere deserve a vote on these issues," Reid wrote. "We owe them a vote, and I will make sure they get a vote." He added, "Only those who are afraid of a free and open debate would try to block it or shut it down."

With guns, as with gay rights and immigration, Washington has observed in Reid an evolution — less flip-flops than slow dances to the left — that reflect shifting attitudes not only in his Democratic conference but also in Nevada, where Democrats have gained an edge in the last decade. Voter registration in the state has become increasingly Democratic as its population has swelled, and Barack Obama won the state twice, the only Democrat besides Bill Clinton to win the state in the last 40 years.

"Harry Reid is the most calculating individual I have ever covered in politics," said Jon Ralston, editor of Ralston Reports, who has covered Nevada politics for three decades. "If he is making the right move for his members, he is making the right move for himself."

Reid voted proudly against an assault weapons ban in both 1993 and 2004, even as most Senate Democrats voted for it, and voted for a successful 2005 measure that limited lawsuits against gun manufacturers and dealers for negligence. He has also long supported the NRA.

But now, in a demonstration of his loyalty to President Barack Obama, Reid is helping him pursue his agenda for stemming gun violence. Many of the more senior members of his caucus, notably Sen. Dianne Feinstein of California, also want these votes.

"He is doing what a leader needs to do," said Sen. Patty Murray of Washington, one of Reid’s protegees in the Senate, "to move the caucus forward so it stays in tune with where the American people are."

Reid, aides said, is also motivated by both the personal angst he felt over the killing of 20 schoolchildren in Newtown, Conn., last year, as well as the anger he feels toward the NRA, which was widely expected to endorse in his 2010 re-election campaign but then declined to do so.

After the Senate returns from its recess next week, it will consider a bill that would expand background checks and increase penalties for so-called straw purchases, in which someone buys a gun for another person who is unable to buy one. Reid opted not to include in the bill a ban on assault weapons and high-capacity ammunition magazines but plans to hold a separate vote on both measures. His hope was to not let the less popular measures jeopardize passage of the expanded background checks.

Reid is almost certain to vote in favor of at least some of the new gun safety measures, if not all of them.

It would not be the first time Reid shifted his position on a significant public policy issue. For example, in 1993, Reid co-sponsored legislation that would have stripped the citizen rights from babies born to illegal immigrant mothers, and vigorously denounced immigrants from the floor. The bill did not make it out of the Judiciary Committee.

Just over a decade later, Reid apologized for the legislation, which he called "the low point of my governmental career," and became a proponent of the so-called Dream Act, which would give a pathway to citizenship to some children of illegal immigrants. Immigration reform was a centerpiece of his 2010 re-election campaign, against the advice of many of his political strategists.

Similarly, Reid voted for a 1993 measure that institutionalized the "don’t’ ask, don’t tell" policy for gay members of the military. But by 2009 he became an opponent of the policy. That year, Reid, asked to support a moratorium on the practice, said he would go further and press for its repeal, an offhand statement that ignited the repeal efforts in his chamber.

During the 2010 lame-duck session, Reid repeatedly and vociferously pressed for the repeal, including in an emotional floor speech in which he said: "Discrimination has never served America well. When it applies to those who serve America in the armed forces, it is both disgraceful and counterproductive."

When a move to repeal the policy failed on a procedural vote, Sen. Kirsten Gillibrand, D-N.Y., went back to Reid and begged for a second vote, which he delivered, and the measure passed, with bipartisan support.

Like many Democrats — and a few Republicans — Reid has also re-evaluated his views on same-sex marriage. For years, Reid repeatedly said that "marriage is between a man and a woman," and he voted for the Defense of Marriage Act of 1996, which denied federal marital benefits to gay couples. Only recently has Reid started saying that gay and lesbian couples have the right to marry; he co-signed the amicus brief urging the Supreme Court to find the law unconstitutional. Gay marriage was banned in Nevada by initiative a decade ago and there is now a bill to repeal the ban before the state Legislature.

"He deserves a lot of credit for having the guts to stick his neck out," Bloomberg, an independent, wrote in an email, "especially on guns and immigration when others are worried about special-interest politics."

Reid’s biggest struggle is to balance the needs of vulnerable Democrats who are up for re-election — especially moderate, long-serving members who are largely institutionalists — with the agendas of newcomers who lean farther to the left.

Since 2006, as larger-than-life Democrats like Sens. Robert C. Byrd, Edward M. Kennedy and Daniel K. Inouye have died, Democrats have had a big influx of members pressing Reid toward a more aggressive and often liberal stance.

"My role is to listen to every single member of the caucus and understand where they are coming from," Reid said by email. "Everyone won’t be one hundred percent happy all the time, but I try to make sure that all voices have input on the decision-making process, and understand the steps we are taking."

This conflict came to the fore during discussions of changing the filibuster rules; Reid ultimately chose an approach that placed fewer limitations on a minority party’s ability to filibuster than the newer members wanted.

"The Democratic caucus is a progressive caucus," said Sen. Sherrod Brown of Ohio. "I think he leads us mostly the way we want to go. I think we all learned lessons on the filibuster."

Reid remains an enigma of sorts in Washington, a quiet force whose voice is often barely audible, who skips Sunday talk shows in favor of church and lunch with his wife and remains, by all accounts, gaga for her after decades of marriage.

He is a man who relies on his members to do the routine prep work on bills until the time comes to marshal votes. He also prefers brief conversations to lengthy ones.

"There isn’t one senator who doesn’t know when you talk to Harry Reid on the phone you better say what you have to say fast," Murray said. "Many senators have found themselves talking after he has already hung up."

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