A bipartisan proposal to expand background checks for gun buyers appeared on Tuesday to be short of the 60 votes needed to clear the Senate, as supporters scrambled to save the centerpiece of President Barack Obama's effort to reduce gun violence.
Debate on the plan forged by Democrat Joe Manchin of West Virginia and Republican Pat Toomey of Pennsylvania continued in the Senate for a second day on Tuesday, with no vote scheduled.
The plan needs 60 votes to overcome procedural hurdles erected by Republican opponents in the 100-seat Senate, where Democrats control 55 seats. Several Democrats from conservative, gun-friendly states could vote against the plan, leaving supporters in need of Republican support.
"We haven't voted on it because supporters don't have the votes to pass it," said Republican Senator Charles Grassley of Iowa, an opponent of the proposal, who argued that it would not have stopped the massacre of 20 school children and six adults in Newtown, Connecticut, in December, or other mass shootings.
The plan to extend criminal background checks to online and gun-show sales has been seen as Obama's best hope for meaningful gun-control legislation in response to the Newtown shootings.
Opinion polls show that more than 80 percent of Americans favor expanded background checks, but the amendment has drawn opposition from the National Rifle Association gun lobby, and most Republicans in the Democrat-led Senate. Even if it were able to clear the Senate, it would face a rough ride in the Republican-led House of Representatives.
The NRA has warned lawmakers it will include their vote in the ratings it compiles on them and sends to its 4 million members. Mayors Against Illegal Guns, a gun-control group backed by New York City Mayor Michael Bloomberg, has also said it will rate members of Congress based on their votes.
Administration officials, including Vice President Joe Biden, have helped Manchin and Toomey lobby senators. Relatives of victims of Newtown also visited Washington last week and had several emotional meetings with lawmakers in which they urged them to support expanded background checks and other measures.
Among Republicans, only Toomey, Susan Collins of Maine and Mark Kirk of Illinois have committed to support the Senate proposal. John McCain of Arizona said on Sunday he was "favorably disposed" to it.
Several key senators from states where hunting and guns are popular - including Democrats Mark Pryor of Arkansas, Heidi Heitkamp of North Dakota and Mary Landrieu of Louisiana - remain uncommitted.
Democratic Senate Leader Harry Reid said he hoped to reach an agreement with Republicans on Tuesday on a schedule for votes on the proposal and other amendments, including those by Republican opponents of the bill.
"There are disagreements as to what we should do with gun legislation, if anything, and I understand that," Reid said on the Senate floor. "It's time to begin processing these amendments. I hope that we will be able to reach an agreement earlier rather than later."
Whatever is eventually agreed to appears likely to fall far short of what Obama sought immediately after the Newtown shootings. The bill also includes tighter restrictions on gun trafficking and more funding for school security.
Amendments to add restrictions such as a ban on rapid firing "assault" weapons like the one used in Newtown and limits on the capacity of ammunition magazines appear to have little chance for approval.
Opponents of the Manchin-Toomey plan and some other elements of the legislation say the proposals are an example of government overreach that would infringe on the constitutional right to bear arms.
"Manchin-Toomey would impose new obligations on law-abiding gun owners," Grassley said.
Manchin and Toomey, both conservatives and strong proponents of gun rights, have argued that their proposal would simply make it more difficult for criminals and the mentally ill to buy guns.
Their amendment includes sweeteners for gun-rights supporters, including a provision that would make licensed interstate sales easier and ban the creation of a gun registry, one of the frequent fears cited by groups such as the NRA.