WASHINGTON -- A bipartisan deal seems imminent on expanding background checks to cover more gun buyers, an agreement that could build support for President Barack Obama's drive to curb firearms violence in the wake of the Connecticut elementary school massacre.

Meanwhile, the Senate is ready for an opening vote on gun control measures as Majority Leader Harry Reid set a roll call for Thursday on starting consideration of the firearms legislation. Odds are growing that Democrats will win enough Republican support to thwart an effort by conservatives and Senate Minority Leader Mitch McConnell to block the debate from even starting.

While the expanded background checks would close some loopholes that allowed unfettered purchases at gun shows and online, it would exempt transactions like face-to-face, noncommercial purchases, said Senate staffers and lobbyists, who spoke on condition of anonymity to discuss the private talks. Currently, the checks are required only for sales handled through licensed gun dealers.

Tougher restrictions like a ban on assault weapons and high-capacity clips are not even being discussed as they have little popular support in a country weary of gun restrictions and heavy influenced by a powerful gun lobby.

Even if the expanded background checks pass in the Senate where the Democrats hold a slim majority it faces strong opposition in the Republican-controlled House.

"I hope Republicans will stop trying to shut down debate and start engaging in the tough issues we were sent to Washington to tackle," Reid said.

Together, the developments were a boost for gun control advocates battling for restrictions following the Dec. 14 shootings that killed 20 children and six staffers at Sandy Hook Elementary School in Newtown, Connecticut.

Even so, the ultimate fate of gun legislation remains unclear, clouded by opposition from many Republicans and some moderate Democrats in the Democratic-led Senate and the Republican-run House of Representatives. Many critics say the proposal would violate the Second Amendment of the U.S. Constitution which guarantees the right to bear arms and burden law-abiding gun owners.

"We should focus law enforcement resources on the bad guys," said Republican Sen. Ted Cruz of Texas, who is among the Republican senators supporting the use of parliamentary manoeuvrs to block consideration of the gun control legislation.

Sens. Joe Manchin, a West Virginia Democrat, and Patrick Toomey, a Pennsylvania Republican, were expected to announce a background check compromise on Wednesday. Subjecting more firearms purchases to federal background checks has been the chief goal of Obama and gun control supporters, who promote the system as a way to prevent criminals and other risky people from getting the weapons.

After weeks of negotiations, Manchin and Sen. Chuck Schumer, a New York Democrat, told reporters late Tuesday that a gun control agreement was close.

Though many details of the emerging agreement were unclear, Manchin and Toomey are among their parties' most conservative members and a deal could make it easier for some hesitant senators to support the background check measure, at least for now.

Some Republicans might vote to begin debate on the legislation but eventually oppose the measure on final passage. Other parts of Obama's gun effort already seem likely to face defeat, including proposed bans on assault weapons and high-capacity ammunition magazines.

The gun legislation Reid wants the Senate to debate would extend the background check requirement to nearly all gun sales. Assuming the deal between Manchin and Toomey is completed, Reid would try to replace that language with their agreement once debate begins, a move that would require a vote.

The overall gun bill also tightens federal laws against illegal gun sales and slightly increases federal aid for school safety.

Thirteen conservative senators have signed a letter saying they will block consideration of the measure, and McConnell said he will back that move. That will force Democrats to round up 60 votes to overcome the conservatives.

At least eight Republican senators have said they want to begin debate or have indicated a willingness to consider it, a number that would be expected to grow if the background check agreement proves successful.

Some moderate Democrats were remaining noncommittal and could oppose opening the gun debate. They include Sens. Mark Begich of Alaska and Mark Pryor of Arkansas, who are seeking re-election next year. There are 53 Senate Democrats and two independents who lean Democratic.

Amid the manoeuvring, relatives of some Newtown victims have been lobbying Congress to support gun curbs. And Obama has been calling senators from both parties to push for the gun bill.

"People should listen to what we have to say and move the debate forward," said Mark Barden, who lost his 7-year-old son, Daniel, at Sandy Hook. "It's not just about our tragedy. Lots of kids are killed every day in this nation. We have to help lead the change."

The National Rifle Association, the influential gun rights lobbying group, opposes Obama's effort and is urging its members -- it claims nearly 5 million -- to tell lawmakers of their opposition.

Counteracting that has been an effort by Mayors Against Illegal Guns, one of whose leaders is billionaire New York Mayor Michael Bloomberg. It said it will keep track of key gun-related congressional roll calls and make the information available to voters and contributors -- a tactic long used by the NRA and other groups.