A bipartisan group of Senate negotiators signaled Tuesday night that it has reached a deal in principle on expanding background checks to include more gun sales, in what was widely seen as the major sticking point on the biggest gun control legislation to reach the floor since 1994.

Sens. Joe Manchin III, D-W.Va., and Patrick J. Toomey, R-Pa., said they would hold a news conference at 11 a.m. Wednesday to discuss the details of the tentative deal, which was reached with the support of Sen. Charles E. Schumer, D-N.Y., and could entice a significant number of other lawmakers to sign on. Schumer told reporters that some details still needed to be worked out but that “we’re closer than we’ve ever been.”

“Tomorrow we hope to finalize it,” Manchin said Tuesday night. A Toomey spokesman added, “Sens. Toomey and Manchin continue to work on final details, but they appear close to a deal.”

The plan is expected to stop short of language currently in the bill that would require background checks on nearly all gun sales, including between private parties. Instead, Toomey aides said, the proposal would require background checks for private sales at gun shows and on the Internet, two areas that are currently exempt.

Nevertheless, support for the plan by Toomey, a reliable conservative with a top rating from the National Rifle Association, would be a major victory for Senate Democrats and President Barack Obama, who have stepped up their calls in recent days to ensure that criminals and the mentally ill do not have easy access to guns.

Even as the negotiators worked to finalize their deal on background checks, Senate Majority Leader Harry Reid, D-Nev., set up a procedural vote to test a threat by 13 Republicans — in addition to Minority Leader Mitch McConnell of Kentucky — to filibuster the overall measure.

Reid filed cloture Tuesday night on the motion to proceed to the bill (S 649), which, besides expanding background checks, would impose new criminal penalties on gun traffickers and authorize new funding for school security measures. Reid has also promised to allow amendments on other gun proposals.

“It would be a real slap in the face to the American people not to do something on background checks, on school safety, on federal trafficking, which everybody thinks is a good idea,” he said.

The cloture vote — if it still occurs in the wake of an agreement between Manchin and Toomey — dials up the pressure on Republicans to allow debate on the bill, even if they oppose it. But even before Reid’s announcement of the vote, there were signs of a significant fissure within the GOP ranks on the question of a filibuster.

At least seven Republicans expressed skepticism Tuesday about their GOP counterparts’ plan to prevent a debate. The seven are Sens. Jeff Flake and John McCain of Arizona, Johnny Isakson of Georgia, Susan Collins of Maine, Kelly Ayotte of New Hampshire, Tom Coburn of Oklahoma and Lindsey Graham of South Carolina.

Their position stands in sharp contrast to the filibuster threat voiced by the 13 other Senate Republicans who sent a letter to Reid on Monday in which they vowed to block a motion to proceed to “any legislation that would serve as a vehicle for any additional gun restrictions.”

McConnell has taken a slightly different stance by saying he opposes cutting off debate on the gun legislation as it is currently written, though he could presumably choose to support it after the deal reached by Manchin and Toomey.

“I don’t fear the debate,” Graham said Tuesday. “I welcome the debate. That’s just me. I think I’m speaking for a wide number of people in the conference.”

If Graham and other anti-filibuster Republicans join with a united or near-united Democratic caucus, the 60-vote threshold on the motion to invoke cloture would be met.

While the whip count on the cloture vote appears to favor Democrats on the procedural vote, Reid warned that he may lose some of his own members — at least one of whom, Sen. Mark Pryor of Arkansas, confirmed Tuesday that he is considering joining the GOP filibuster. Pryor faces one of the toughest re-election campaigns in the Senate next year.

“I want to see how this develops,” Pryor said, noting that he opposes the legislation as it is currently written. “I know that many senators are working to try to get some sort of compromise.”

Even if the cloture vote fails Thursday, Reid said he would continue to the bill under newly approved Senate rules that would allow him to bypass the procedural maneuver but require him to offer each party two amendments to the bill.

“I also alert everyone that if they don’t help me invoke cloture on this bill, we’re going to vote on these things anyway,” Reid told reporters. “It’ll take a little bit of time, but as I’ve said for months now, the American people deserve a vote on background checks, on federal trafficking, on safety in schools, on the size of clips and, yes, assault weapons, and of course mental health.”

Humberto Sanchez and Meredith Shiner contributed to this report.