* West Virginia's Manchin, Pennsylvania's Toomey negotiating
* Democrats' recruiting of gun-rights candidates complicates Obama's proposal
* For Democrats, a pragmatic decision in 2006
By Thomas Ferraro
WASHINGTON, April 8 (Reuters) - The fate of one of President Barack Obama's key gun-control proposals appeared on Monday night to be in the hands of two U.S. senators: one Democrat, the other Republican, both of them longtime opponents of restrictions on guns.
Democratic Senator Joe Manchin of West Virginia and Republican Senator Pat Toomey of Pennsylvania are seeking a compromise on expanding background checks for prospective gun buyers. The proposal appears to be Obama's best hope for meaningful gun-control legislation in the wake of the December massacre of 20 children and six adults at a school in Connecticut, where the president spoke Monday evening to a crowd that included some victims' families.
If Manchin and Toomey can't strike a deal, Obama's calls to require nearly all gun buyers to submit to background checks for criminal records and mental health problems likely will fail, Senate aides said on Monday.
Another centerpiece of Obama's efforts, a renewal of a ban on the sale of military-style assault weapons, appears unlikely to be approved, said Senate Majority Leader Harry Reid, a Nevada Democrat. A proposed 10-bullet limit on ammunition magazines also appears to be in trouble, he has said.
Obama, who has called the day of the Newtown massacre the worst of his presidency, made a strong appeal on Monday for action in Congress.
But at this point, Senate aides said, there seems to be strong, bipartisan support for only two of Obama's proposals: a crackdown on gun trafficking and improvements to security at schools.
The talks between Manchin and Toomey - both allies of the National Rifle Association, the nation's largest gun-rights group - symbolize why the background checks proposal is not a sure thing in Congress, even though polls have indicated that it is backed by 80 to 90 percent of Americans.
Toomey is pushing for expanded background checks even as 13 conservative Republican senators are threatening a filibuster to delay votes on any gun-control bills.
And Manchin represents how Democrats' move in recent years to embrace candidates who strongly support gun rights is complicating the Democratic president's push for gun restrictions.
Manchin is among eight Senate Democrats rated as friendly to gun rights by the NRA to have joined the chamber since 2006.
That was when the Democratic officials - fed up with losing congressional races to Republicans in rural and conservative states such as Montana, Virginia, North Carolina, Alaska and West Virginia - stepped up its recruiting of pro-gun candidates.
The strategy worked.
Democrats won control of the U.S. Senate and House of Representatives in 2006 and increased their majorities in 2008, when Obama captured the White House.
The House fell back to Republicans in 2010, but not before conservative Democrats such as Manchin helped Obama get his healthcare overhaul through Congress.
But now, those conservative Democrats have become speed bumps for several of Obama's gun-control proposals - particularly his call to ban assault weapons, which both sides see as unlikely to pass even the Senate, where Democrats control 55 of the 100 seats.
On expanding background checks, "Manchin is upbeat about their chances of success," said one Senate aide, who offered an assessment of her own: "50-50."
SEEING A 'SLIPPERY SLOPE'
Reid, who schedules floor votes in the Senate, had planned to begin consideration of gun legislation this week. But that timetable could be pushed to next week to give Manchin and Toomey more time to negotiate, or if Republicans try to delay action, aides said.
Even the gun trafficking plan, which would make it a federal crime for someone banned from having a firearm to buy one, could face problems. The NRA is seeking to revise it in a way that critics say would make it difficult to enforce.
Obama's call to ban rapid-firing assault weapons - a pet cause of Senator Dianne Feinstein, a California Democrat - has gone nowhere in part because even those in Feinstein's party are not solidly behind it.
Several Democrats who have joined the Senate since 2006 have said they could not vote for Feinstein's plan.
They include Manchin, Mark Begich of Alaska and Jon Tester of Montana, a gun-rights Democrat and farmer who won a Senate seat in 2006 by unseating a three-term Republican.
Tester said he sees an assault weapons ban as "a slippery slope" that could lead to other restrictions on gun ownership. He said he favors stepping up efforts to keep guns out of the hands of criminals and the mentally ill.
"That would be the smart thing to do," Tester said.
Begich, another Democrat rated as a friend on Capitol Hill by the NRA, ousted a six-term Republican when he was elected in 2008. Begich is a co-sponsor of a bipartisan bill to clarify when a person loses the right to possess a firearm based on mental illness.
Critics of the bill, which is backed by the NRA, say the measure could make firearms more accessible to the mentally ill.
Manchin, elected to the Senate in 2010, has been trying for months to find a way to expand background checks while addressing gun-rights' advocates concerns that such checks could be used to create registries of gun owners. Gun rights advocates say the government could use such registries to confiscate weapons.
"I think everyone wants to do something that is right and responsible," Manchin said. "I really do."
A POLITICAL CALCULATION
Few Democrats regret the party's decision to recruit gun-rights candidates in recent years.
If senators such as Tester and Begich had not won their seats, many Democrats figure, the seats would be held by pro-gun Republicans - and many of Obama's legislative accomplishments, including the healthcare overhaul and new regulations on Wall Street, would not have happened.
"The profile of the guy who can win in these states is the profile of a guy who is pro-gun," said Jennifer Duffy, who tracks Senate races for the nonpartisan Cook Political Report.
Larry Sabato, who tracks congressional races as head of University of Virginia's Center for Politics, said that in 2006, "Democrats made a practical decision ... to win rather, than to be (ideologically) pure."
Sabato said he now sees some Republicans similarly bending their stances on some issues to try to appeal to more voters, such as easing their opposition to gay marriage and tax increases. (Editing by David Lindsey and Lisa Shumaker)