WASHINGTON — Several Senate Republicans said Tuesday that they would not participate in a filibuster of the first major gun control bill since 1993, as Democrats appeared on the verge of overcoming a blockade threatened by a group of conservatives before a word of debate on the measure was uttered.

Senator Harry Reid of Nevada, the majority leader, said he would schedule an initial showdown vote for Thursday. If backers of the measure can corral at least 60 votes, the Senate will begin consideration of a series of gun safety proposals — strongly supported by President Obama — that would still face a long and difficult journey across the Senate floor.

The bill, which would increase penalties for illegal gun purchase and greatly expand background checks on gun buyers, would again need 60 votes to end the ensuing debate after consideration of contentious amendments, including a renewal of the assault weapons ban. Should it cross that very high hurdle, 51 votes would be needed to get to final passage. Even with Democrats controlling 55 seats, no majority was assured given the resistance of some Democrats from more conservative states who face re-election campaigns next year.

Even as Mr. Reid scheduled a vote, Senators Joe Manchin III, Democrat of West Virginia, and Patrick J. Toomey, Republican of Pennsylvania were very near a deal that would most likely serve as an amendment replacing the background check piece of the measure that Mr. Reid is seeking a vote on. Their measure, which would almost certainly appeal to a broader base of members than the one now at the heart of the debate, would include fewer gun buyers in newly expanded checks, but allow for the record keeping that many Republicans have opposed. The two were expected to announce a deal Wednesday. Mr. Manchin briefed Mr. Reid late Tuesday.