By John Whitesides and Patricia Zengerle
WASHINGTON (Reuters) - Two key senators announced a breakthrough deal on Wednesday to expand background checks for gun buyers, boosting prospects that the Senate will approve at least some of President Barack Obama's proposed gun restrictions.
The agreement by Democrat Joe Manchin of West Virginia and Republican Pat Toomey of Pennsylvania sets the stage for a Senate debate on a gun-control package starting on Thursday, when the Democrat-led chamber is expected to defeat conservative Republican efforts to block the bill from even reaching a vote.
The proposal for expanded background checks appears to be Obama's best hope for meaningful gun-control legislation in the aftermath of the December massacre of 20 children and six adults at an elementary school in Newtown, Connecticut.
Manchin and Toomey, both ardent defenders of gun rights, said their measure would expand criminal background checks for prospective gun buyers to include sales made at gun shows and online, closing a major loophole in a system that analysts say allows as many as 40 percent of gun buyers to avoid checks.
But the agreement, designed to keep guns out of the hands of dangerous criminals and the mentally ill, left significant exemptions for "temporary transfers" of weapons or private sales among friends and family members. No background checks would be required in such transactions.
More controversial parts of Obama's plan - such as a ban on rapid-firing "assault" weapons like the one used in Connecticut and limits on the capacity of ammunition magazines - appear to have a slim chance of clearing the Senate.
The bipartisan deal on background checks does not guarantee that it will clear the Senate, where Obama's Democrats control 55 of the 100 seats but many Democrats, like Manchin, are strong supporters of gun rights.
Toomey acknowledged as much during the news conference he and Manchin held on Wednesday, saying that if amendments are added to the bill that he believes infringe on gun owners' rights, he would not support the bill even if it still included the background checks compromise.
If the legislation does clear the Senate, it would still face a tough road to approval in the Republican-controlled House of Representatives.
"Today is just the start of a healthy debate that must end with the Senate and House hopefully passing these common sense measures and the president signing them into law," Manchin told reporters.
He said Democratic leaders have promised the background checks agreement will be the first amendment offered to the Senate's gun-control bill.
The deal also created a commission that would study the causes of mass violence in the United States, examining images of violence in media and video games as well as issues such as school safety, guns and mental health.
Six members would be appointed by the Senate majority leader, currently Nevada Democrat Harry Reid, and six by the House speaker, currently Republican John Boehner.
Obama, who has been pushing hard for Congress to produce the first major gun-control legislation in nearly two decades, had called negotiators from both parties on Tuesday, the White House said. Manchin said the senators also had been in touch with key stakeholders such as the powerful National Rifle Association gun lobby.
The president's plan has been the focus of intense lobbying by gun-rights supporters such as the NRA, and by gun-control advocates such as Mayors Against Illegal Guns, a group backed by Michael Bloomberg, the media magnate and New York City mayor.
The NRA called the agreement's rejection of the broader "universal" background checks pushed by Obama "a positive development," but also said a "meaningful and serious" solution to gun violence was needed that addresses crime and mental health issues.
The deal also won cautious support from some gun-control advocates, including Bloomberg and the group formed by former Congresswoman Gabrielle Giffords, who was severely wounded in a 2011 mass shooting in Arizona.
"Our bipartisan coalition of more than 900 mayors strongly supports this bill," Bloomberg said in a statement.
Obama, who has described the day of the December 14 Newtown shooting as the worst of his presidency, has used a campaign-style approach to his gun-control efforts.
Several family members of victims from the shootings at Sandy Hook Elementary School in Newtown flew to Washington on Air Force One on Monday to urge lawmakers to take action, and First Lady Michelle Obama was scheduled to give a speech on guns on Wednesday.
Manchin's participation in the deal on background checks could bolster the prospects for the legislation. The freshman senator from West Virginia represents a state where gun ownership has long been passionately protected and where attempts to regulate guns have been strongly opposed.
Toomey, a conservative Republican from Pennsylvania, said he did not believe expanding background checks amounted to gun control. "It's common sense," he said. "What matters to me is doing the right thing, and this is the right thing."
Senators Mark Kirk of Illinois, a Republican, and Chuck Schumer of New York, a Democrat, also participated in the negotiations.
On Thursday, the Senate is scheduled to hold its first vote on whether to take up a gun-control bill. More than a dozen conservative Republican senators have threatened a filibuster aimed at preventing consideration of any gun restrictions.
But with public opinion polls showing up to 90 percent of Americans favor expanded background checks, other Republicans have said Obama's proposals should get a Senate vote.
The measure likely to pass the Senate could also include funding for school security and tighter restrictions on gun trafficking.
That package would fall short of what Obama had pressed for, but would be far more extensive than the NRA would like.
(Additional reporting by Richard Cowan and Sam Youngman; Editing by David Lindsey and Claudia Parsons)