WASHINGTON — The Senate’s top Democrat has set Congress’ first showdown vote for Thursday on President Barack Obama’s gun control drive as a small but mounting number of Republicans appear willing to buck a conservative effort to prevent debate from even beginning.

Majority Leader Harry Reid of Nevada announced his decision Tuesday as the White House, congressional Democrats and relatives of the victims of December’s mass shooting in Newtown, Conn., amped up pressure on GOP lawmakers to allow debate and votes on gun control proposals.

Meanwhile, participants from both parties said a bipartisan deal was imminent on expanding required federal background checks to gun purchases conducted at gun shows and online. The two chief negotiators, Sens. Joe Manchin, D-W.Va., and Patrick Toomey, R-Pa., were expected to announce the compromise today.

Manchin and Sen. Chuck Schumer, D-N.Y., told reporters late Tuesday that a deal was close. A Toomey aide said the same, speaking on condition of anonymity to discuss closed-door bargaining.

Reid said he did not know if he had the 60 votes he will need to defeat the conservatives’ roadblock. But at least eight Republicans have said they want to begin debate or have indicated a willingness to consider it: Sens. John McCain of Arizona, Lindsey Graham of South Carolina, Tom Coburn of Oklahoma, Johnny Isakson and Saxby Chambliss of Georgia, Susan Collins of Maine, Roy Blunt of Missouri and Mark Kirk of Illinois.