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. Twenty first-graders and six educators were gunned down at Sandy Hook Elementary School, turning gun control into a top-tier national issue.

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.

An agreement on background checks - the cornerstone of Obama's plan to restrict firearms - could boost bipartisan support for the overall effort, at least initially, because Manchin and Toomey are among their parties' most conservative members. But the ultimate fate of gun legislation remains unclear, clouded by opposition from many Republicans and moderate Democrats in the Democratic-led Senate and the GOP-run House.

The emerging deal would expand required federal background checks to sales at gun shows and online, but exempt transactions like face-to-face, non-commercial purchases, said several Senate aides and lobbyists who spoke on condition of anonymity to describe the private talks.

Currently, the checks are required only for sales through licensed gun dealers.

On the Senate floor, Reid pointed to a poster-sized photo of a white picket fence that had slats bearing the names of the Newtown victims.

"We have a responsibility to safeguard these little kids," said Reid, D-Nev. "And unless we do something more than what's the law today, we have failed."

"We don't have the guts to stand up and vote yes or no? We want to vote maybe? Tell that to the families in Newtown" and other communities where there have been mass shootings, said Senate Judiciary Committee Chairman Patrick Leahy, D-Vt.

But Sen. Ted Cruz, R-Texas, one of 13 conservative senators who signed a letter promising to try blocking debate, said the Senate bill puts "burdens on law abiding citizens exercising a constitutional right." He said none of its provisions "would have done anything to prevent the horrible tragedy of Sandy Hook."

Obama was calling senators from both parties Tuesday to push for the gun bill, according to a White House official.

A Senate vote to begin debating the guns package would mark a temporary victory for Obama and his allies.

Some Republicans, though eager to avoid blocking debate, could vote against the measure on final passage. Coupled with resistance by leaders of the GOP-run House to main parts of Obama's effort - including bans on assault weapons and high-capacity ammunition magazines - the ultimate outcome seems shaky for Democrats.

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.

But some moderate Democrats are remaining noncommittal and might oppose opening the gun debate, including Sens. Mark Begich of Alaska and Mark Pryor of Arkansas, who are seeking re-election next year.

Begich declined to directly state his position and said of Alaskans, "We like our guns."

There are 53 Senate Democrats and two independents who lean Democratic.

In a written statement, Sen. Mike Lee, R-Utah, said conservatives want to prevent Obama from rushing the legislation through Congress "because he knows that as Americans begin to find out what is in the bill, they will oppose it."

Senate Minority Leader Mitch McConnell, R-Ky., has said he will join the conservatives' attempt to block debate.

The bill would expand required federal background checks to nearly all gun transactions - a provision Reid would try to replace with the completed agreement between Manchin and Toomey. It would also stiffen penalties for illegal firearms trafficking and provide a small boost in school safety aid.

Eleven Sandy Hook family members representing eight of the shooting victims were on Capitol Hill to lobby senators from both parties for gun legislation, including Isakson.

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

Some relatives had breakfast with Vice President Joe Biden at his residence in the Naval Observatory. Later, Biden spoke to law enforcement officials at the White House and told reporters that conservatives would not succeed in blocking debate.

"This is not one of these votes that they block a vote and somehow we're going to go away," Biden said. "The American public will not stand for it."

The president's gun-control proposals have hit opposition from the National Rifle Association, which was using the Internet and emails to urge its members - it claims nearly 5 million - to tell members of Congress of their opposition.

In GOP-heavy Louisiana, where Democratic Sen. Mary Landrieu is seeking re-election next year, the NRA wrote to its members, "Please contact Senator Landrieu and encourage her to oppose this anti-freedom legislation."

Counteracting that has been an effort by Mayors Against Illegal Guns, one of whose leaders is billionaire New York City Mayor Michael Bloomberg.

The group was running a television ad in Pennsylvania asking voters to contact Toomey and tell him to back expanded background checks. It also 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.

Associated Press writers Nedra Pickler, Jim Abrams, Andrew Miga and Henry C. Jackson contributed to this report.