The Senate could vote as early as Wednesday on expanded background checks for gun buyers, but leading lawmakers said on Sunday it was uncertain whether the contentious proposal could gather enough support to pass.
The compromise legislation offered by Democrat Joe Manchin of West Virginia and Republican Pat Toomey of Pennsylvania is seen as President Barack Obama's best hope for meaningful gun-control law in the wake of last year's mass shooting in Newtown, Connecticut.
"We expect the vote this week. Wednesday is probably the most likely day for the Manchin-Toomey alternative," said Toomey on CNN's "State of the Nation."
"It's an open question whether we have the votes. I think it's going to be close."
The legislation calls for expanded criminal background checks of gun buyers to be required for commercial sales, including those made at gun shows and online. However, sales by private persons would be exempt.
The background checks, intended to prevent criminals and the mentally ill from buying guns, have met stiff opposition from conservative Republicans who argue that the rule infringes on the constitutional right of law-abiding Americans to own guns.
Toomey and Manchin, both ardent defenders of gun rights, have dismissed this assertion.
"This bill, if you are a criminal and have been mentally adjudicated, you may not like it," said Manchin.
Background checks have the support of Senator John McCain, the Arizona Republican who was the party's presidential candidate in the 2008 election.
"I am very favorably disposed towards that. Eighty percent of the American people want to see a better background checks procedure," McCain said on CNN.
But gun-control legislation generally does not have the support of conservative Republicans, including Senator Marco Rubio of Florida, considered in some circles as a potential 2016 presidential candidate.
Rubio said the focus of the legislation should be on curbing gun violence, not restricting gun ownership.
"Gun laws are highly ineffective in terms of protecting the right of law abiding citizens to possess weapons, which is a Second Amendment guarantee, (and) keeping the guns out of the hands of criminals," said Rubio on NBC's "Meet the Press."
"I think this is a missed opportunity to have an honest and open conversation in this country about why these horrifying things are happening."