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.

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.

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