The success of President Obama’s starkly liberal second-term agenda will rest largely on the shoulders of Senate Majority leader Harry M. Reid, who has been a rock-solid political ally and a valued legislative tactician for Obama during his first term.

But for the first time since Obama became president four years ago, his political interests and Reid’s may be diverging. Not so much because there is huge disagreement on the president’s agenda, but because helping Obama may hurt vulnerable Democrats in the Senate.

Reelected and unconcerned about ever having to face voters again, Obama seems determined to push a far-reaching agenda — on guns, climate change and gay rights, among other topics — that looks toward his presidential legacy. Reid (D-Nev.), significantly more encumbered, must worry about how to protect 20 Democratic-held Senate seats that will be up for grabs in 2014, while Republicans are defending only 14 spots.

For some Democrats up for reelection next year, supporting the president will be politically treacherous terrain, and no issue may capture that disconnect better than gun control.

Sens. Max Baucus (Mont.), Tim Johnson (S.D.), Mary Landrieu (La.), Mark Udall (Colo.) and Mark Begich (Alaska) face reelection battles in states where gun control is politically unpopular, making their potential votes on the Obama proposals problematic.

How the NRA exerts influence over Congress

Even if those Democrats vote against some or all of the proposals, they are likely to find themselves tied to the president’s effort to rein in gun rights — just as dozens of House Democrats voted against Obama’s health-care legislation but were still attacked over the issue in their campaigns in 2010.

This week, Senate Minority Leader Mitch McConnell (R-Ky.) signaled his belief that gun rights is still a winner. “Our Second Amendment rights are under attack, and I am ready to do whatever it takes to stand up for our freedom,” he wrote in a fundraising pitch, a day after his campaign manager sent a similar missive to potential donors with an ominous warning: “The gun-grabbers in the Senate are about to launch an all-out assault on the Second Amendment.”

But the president — keenly aware of how national polls show a tilt toward stronger gun-control laws following the Dec. 14 shootings at an elementary school in Newtown, Conn. — has made new gun restrictions a priority.

The Senate Judiciary Committee will hold hearings on new gun proposals this month, and on Thursday, Sen. Dianne Feinstein (D-Calif.) plans to formally introduce legislation that would ban assault weapons and limit ammunition clips to 10 rounds.

Aides say there probably will be three main packages of gun-control legislation over the coming months, with one anchored by the assault-weapons ban, which is considered the most difficult. Another set of proposals will include an effort to establish universal background checks for all firearm sales in retail stores, gun shows or private exchanges. The other piece would include limiting the size of gun magazine clips.

Senate Democrats have yet to decide the order: whether to start with background checks — their most likely victory — and try to build momentum, or to save that for the final piece so the effort ends on a positive note.

Reid has remained silent on the individual gun-control proposals. His prepared statement last Wednesday, after Obama announced the proposed legislation, left gun-control advocates and gun rights supporters parsing each sentence: Some focused on Reid’s hailing of the president’s “thoughtful recommendations,” while others noted his vague promise that “the Senate will consider legislation.”

Some of Reid’s fellow Democrats are worried. Feinstein said she had a private conversation with Reid to voice her displeasure after he told a Nevada television station that, given the current political environment, it might be futile to move an assault-weapons ban through Congress. “You have to try, you can’t sit back and just let the gun organizations call public policy,” Feinstein said in an interview last week.

On Tuesday, Reid sounded more open to a bold approach. “This is an issue that we’re not going to run from,” he told reporters. “It’s an issue we need to talk about. . . . It may not be everything everyone wants. But I hope it has some stuff in there that’s really important.”

Reid’s relationship with NRA

Current and former advisers say Reid’s uneasy relationship with the National Rifle Association could make him open to more stringent gun laws than in prior years, as he balances the chance to pass historic legislation with the need to protect some of his more vulnerable incumbents from tough votes on gun control.

In March 2010, Wayne LaPierre, the executive vice president of the NRA and the president’s gun-control nemesis, made a politically helpful appearance for Reid at the opening of a new shooting range in southern Nevada.

LaPierre heaped praise on Reid, who was in deep trouble going into his reelection contest.

“I also want to thank you, Senator, for your support every day at the federal level for the Second Amendment and for the rights of American gun owners,” LaPierre said.

But five months later, the NRA declined to endorse Reid, deciding instead to stay neutral in the election. Reid won but considered the NRA’s silence a betrayal.

“He doesn’t forget,” said one Reid adviser who spoke on the condition of anonymity to describe Reid’s reaction.

On its face, the NRA endorsing a Senate Democratic leader in a tough reelection bid seems outlandish, but Reid is not an ordinary Democratic leader. He grew up in a tiny Nevada mining town, Searchlight. He and his three brothers bonded over shooting dove during childhood, and he still owns a 12-gauge shotgun.

A right-handed triggerman, he nailed two clay birds out of six shots fired at the 2010 event with LaPierre. The 2,900-acre shooting range outside Las Vegas that was the scene of the event was built largely through land-procurement legislation backed by Reid, along with $61 million in federal funding.

Reid has a lifetime grade of “B” from the NRA, and the group’s political action committee has donated more than $9,000 to Reid’s past two Senate campaigns. Moreover, as majority leader since 2007, Reid has not advanced any gun-control measures, with his power growing by helping elect other pro-gun Democrats in states such as Alaska, Colorado, Missouri and Montana.

Even when Democrats held 59 and 60 seats in 2009, gun laws were only eased. Legislative amendments won wide approval that loosened gun restrictions in national parks and on Amtrak.

The non-endorsement

By spring of 2010, Reid’s national profile as one of the Democrats’ main combatants in Washington’s partisan wars had made him deeply unpopular in Nevada. Still, his campaign made a pitch to the NRA, touting his track record on guns and noting that opposing Reid would be a big mistake: Had Reid lost, he probably could have been succeeded as majority leader by Richard J. Durbin (D-Ill.) or Charles E. Schumer (D-N.Y.), both of whom hold lifetime “F” ratings from the NRA.

Sharron Angle, a tea party activist, pulled off an upset win in the GOP primary and quickly stumbled on several policy issues, giving Reid an opening. And Reid’s camp believed a late-summer endorsement was coming from the NRA, which would have been a huge boost to the senator.

Instead, the group issued a surprise non-endorsement in late August, citing Reid’s support for Obama’s Supreme Court justices as a reason to back away from LaPierre’s effusive praise for the senator.

The NRA did not return requests for comment about the 2010 race. Reid’s advisers think the reversal was the result of pressure from the many conservative activists who are members of the gun lobby, but Reid won by a comfortable margin and will be central to the outcome of the coming debate.

The question is which Harry Reid will emerge.

He could be the man who, in the heat of his 2010 reelection campaign, appeared with LaPierre and compared recreational shooting to “baseball or football or soccer. These are programs to bring us together.”

But some of his advisers suspect that the gruesome nature of the elementary school shootings in Newtown, combined with the NRA’s 2010 snub, may push Reid in a different direction.

“No policy can prevent a determined madman from committing a senseless act of violence,” he said in a Senate floor speech after the school massacre. “But we need to accept the reality that we are not doing enough to protect our citizens.”

Ed O’Keefe contributed to this report.