WASHINGTON — The fate of a bipartisan Senate effort to subject more firearms buyers to background checks remained uncertain Monday as seven Republicans amenable to a gun control debate were still resisting such an expansion.
On an initial showdown over the gun control bill last week, 16 Republicans voted to reject a conservative effort to derail the measure, a roll call that allowed debate on the legislation to begin. Gun control supporters are hoping they can get enough votes from this group to help win approval for expanded background checks, the cornerstone of the effort by President Obama and others to reduce firearms violence.
So far, seven of the 16 have said they will oppose the bipartisan background check proposal, or are leaning toward doing so. Four said they will support it or are likely to, and the remaining members of that group have not indicated a position.
The Senate will start debate Tuesday on an amendment by Senators Joe Manchin, Democrat of West Virginia, and Patrick Toomey, Republican of Pennsylvania, that would expand background checks to cover transactions at gun shows and the Internet. The system, aimed at stopping criminals and others from getting firearms, now covers only sales handled by licensed gun dealers.
‘‘The Toomey-Manchin proposal, while well-intentioned, is not a solution to illegal gun violence. We already have major holes in the current’’ background check system, Senator Lindsey Graham, Republican of South Carolina, said Monday in a written statement. Graham was among the 16 who voted to allow the gun control debate to begin.
Two Democrats, both facing reelection next year in GOP-leaning states, voted against beginning the gun control debate last week.
Spokesman Devon Kearns said that Senator Mark Begich, Democrat of Alaska, is still reviewing the amendment, while aides to Senator Mark Pryor, Democrat of Arkansas, did not immediately return e-mails and a phone call. Some Democrats from conservative-leaning states are expected to oppose the Manchin-Toomey plan.
There are 53 Democrats and two-Democrat-leaning independents in the Senate. Gun control advocates will need 60 votes for the background check proposal to survive.
On Sunday, Republican Senator Susan Collins of Maine issued a statement saying that she would vote for the Manchin-Toomey compromise, which exempts private gun sales.
The plan would ‘‘strengthen the background check system without in any way infringing on Second Amendment rights,’’ Collins said. But Collins took a wait-and-see approach on the entire package, saying ‘‘it is impossible to predict at this point the final composition of the overall legislation.’’
Arizona Senator John McCain has said he is leaning toward supporting the Manchin-Toomey compromise. It was in McCain’s home state that a gunman with schizophrenia shot Representative Gabrielle Giffords in the head during a 2011 rampage in Tucson that left six people killed.
Even with their support, the vote on the measure — expected as early as Wednesday — will be close. ‘‘It’s an open question as to whether or not we have the votes,’’ Toomey said.
Asked how many votes he thought he had now, Manchin said, ‘‘Well, we’re close. We need more.’’
The measure would require background checks for people buying guns at gun shows and online. Background checks apply only to transactions handled by the country’s 55,000 licensed gun dealers. Private transactions, such as a sale of a gun between family members, would still be exempt.
Advocates say the measures would make it harder for criminals and the mentally ill to get weapons. Opponents argue that the restrictions would violate the Constitution’s right to bear arms and would be ignored by criminals.
Manchin later noted that one gun rights group, the Citizens Committee for the Right to Keep and Bear Arms, has announced support for his plan.
The Manchin-Toomey compromise also was endorsed Sunday by the Independent Firearms Owners Association, a pro-gun group that is smaller and more moderate than the NRA.
The senators’ agreement includes language expanding firearms rights by easing some restrictions on transporting guns across state lines, protecting sellers from lawsuits if buyers passed a background check but later used a gun in a crime and letting gun dealers conduct business in states where they don’t live.