President Obama urged lawmakers to pass his gun-control agenda in an emotional and forceful speech at the White House on Thursday. (The Washington Post)

Gun-control measures that seemed destined to become law after the school shootings in Newtown, Conn., are in jeopardy amid a fierce lobbying campaign by firearms advocates.

Despite months of negotiations, key senators have been unable to find a workable plan for near-universal background checks on gun purchases — an idea that polls show nine in 10 Americans support.

Another provision that garnered bipartisan support — making gun trafficking a federal crime — could be gutted if Republican lawmakers accept new language being circulated by the National Rifle Association.

The failure of those two measures would be a major setback for the White House and its allies, who have acknowledged that two other proposals — bans on assault weapons and high-capacity ammunition magazines — are not politically viable.

President Obama plans to visit a police academy in Colorado on Wednesday to renew an urgency to overhaul the nation’s gun laws that has ebbed in the more than 100 days since the massacre at Sandy Hook Elementary School. Obama and his allies have not been able to leverage nationwide support for the proposals into a will to pass them on Capitol Hill.

How the NRA exerts influence over Congress

And a television ad campaign targeting 13 senators, financed by New York Mayor Michael R. Bloomberg (I) and in its second week on the air, has not swayed enough lawmakers to ensure passage of the background-check measure.

Gun-control proponents are hopeful that senators will soon reach a compromise on background checks even though negotiations are at a standstill. The sides disagree about whether private sales of firearms, such as those between family members or neighbors, would be exempt, as well as how or whether records would be kept.

The NRA voiced support for expanded background checks as recently as 1999. But after the Newtown massacre, it has opposed the idea. NRA officials have argued that the current system is poorly managed and that violators are rarely prosecuted — and they have instilled fear among some key senators that their votes for background checks would have political consequences.

Now some of the same senators targeted by the Bloomberg ads as potential gun-control supporters are showing greater skepticism about expanding checks. The group facing growing pressure from both sides includes a handful of Democrats who will be up for reelection in 2014 in conservative states with strong traditions of gun ownership: Mark Begich (Alaska), Mark Pryor (Ark.), Mary Landrieu (La.), Max Baucus (Mont.), Kay Hagan (N.C.) and Mark R. Warner (Va.).

Pryor, for instance, responded tersely to Bloomberg’s ads, saying last week: “I don’t take gun advice from the mayor of New York City. I listen to Arkansans.”

Several Republicans have threatened to filibuster the bill, which will require a 60-vote majority to pass. And Sen. Jeff Flake (Ariz.), another Bloomberg target and a Republican who may vote for universal background checks, said Sunday on NBC’s “Meet the Press” that it “is a bridge too far for most of us.”

Gun-control supporters have tried in recent days to salvage the legislation.

Sen. Joe Manchin III (W.Va.), one of only seven Senate Democrats with at least an “A” rating from the NRA, has stepped in to try to bridge the divide between senators as well as the interest groups on both sides of the debate, said several aides familiar with the talks.

But, the aides added, there has been virtually no progress since senators left Washington on March 23 for a two-week spring recess. And now, back home, senators are assessing the raw politics of their constituencies to determine which could cost them more in the next election: voting for expanding background checks or doing nothing.

“If there was a secret-ballot vote it would pass overwhelmingly, because from a substantive point of view most of these senators understand that this is the right thing to do,” said Matt Bennett, a gun-control advocate and senior vice president at Third Way, a centrist think tank. “What’s holding them back is pure politics.”

The Republican-led House has put off any consideration of gun-control measures until after the Senate votes. With Senate Majority Leader Harry M. Reid (D-Nev.) planning to begin floor debate on guns next week, NRA lobbyists, as well as Bloomberg’s Mayors Against Illegal Guns group, are reviewing legislative language with senators and their staffs.

On Tuesday, the NRA plans to announce a comprehensive plan for school safety, the results of a process begun in the days following the Newtown shootings when the organization seemed on the defensive. Spokesman Andrew Arulanandam said the plan will “go beyond armed personnel.”

On the separate gun-trafficking measure, the NRA is circulating a proposed revision that critics say would eviscerate the principles agreed to last month by the Senate Judiciary Committee.

The committee’s bill would criminalize all “straw purchases” at licensed gun dealers. But the NRA’s draft language would require law enforcement officials to prove that the straw purchaser had reason to believe the buyer was prohibited from obtaining guns or knew that the buyer intended to commit a crime, according to an analysis of the NRA proposal provided to The Washington Post by the Bloomberg-led mayors group.

Mark Glaze, director of Mayors Against Illegal Guns, said the NRA language would create a “ridiculous” standard for law enforcement officials trying to crack down on trafficking.

The NRA rejected that analysis. Arulanandam said gun-control advocates were misrepresenting a “discussion draft of the type that always circulates in the course of the legislative process.”

Proponents of stricter measures are becoming increasingly fed up with the Senate’s inaction. Rep. Jackie Speier (D-Calif.), a survivor of a mass shooting, said the delays have created “an environment so that cowards can succeed.”

“Ninety-one percent of the American people support a universal background check, and we’ve got members on the House and Senate side that are gutless,” she said. “They know in their heart of hearts that it’s the absolute right thing to do, but they are more concerned about their reelection.”

At the White House on Monday, press secretary Jay Carney said the families of shooting victims “deserve that Congress votes on these measures and not hide behind filibusters.”

Gun-control advocates are trying to match the NRA’s lobbying firepower. Moms Demand Action for Gun Sense in America, a group that sprung up in the days after the Newtown shootings, has been tweeting to lawmakers and visiting their offices to press their case.

Jennifer Fiore, the group’s vice president, recalled that a recent meeting between a mother and an unnamed senior aide to a Senate Republican prompted a sharp, emotional exchange after the staff member repeatedly referred to the recordkeeping provisions in the background-check bill as akin to a national gun registry — a frequent NRA talking point.

“The mom in this office who listened to him talking about registries versus recordkeeping was so fed up with that kind of talk that she got pretty real with him, and at the end of that process I could tell he was listening to us,” Fiore recalled. “Our job is to pop the bubbles that they’re living in and remind them who their constituents are.”

But the exchange also illuminated for Fiore the extent of the NRA’s reach. “They made it into somebody’s office before I got there,” she said.

Tom Hamburger, Sari Horwitz and Scott Wilson contributed to this report.

Discuss this topic and other political issues in the politics discussion forums.