Washington - Prospects for a bipartisan deal to expand federal background checks for gun purchases are improving with the emergence of fresh Republican support, according to top Senate aides.

The possibility that after weeks of stalled negotiations senators might be on the cusp of a breakthrough comes as President Barack Obama and his top surrogates will begin today their most aggressive push yet to rally Americans around his gun-control agenda.

Even though polls show that a universal background check system is supported by nine in 10 Americans, the president has been unable to translate popular support for the measures into legislative momentum on Capitol Hill.

But in a move that could bring along other Republicans as well as Democrats from conservative states who have not yet backed Obama's agenda, Sen. Joe Manchin, W.Va., a key Democratic broker, has spent the past few days crafting the framework of a possible deal with Sen. Patrick Toomey, R-Pa.

Manchin and Toomey are developing a measure to require background checks for all gun purchases except sales between close family members and some hunters, which addresses concerns of some conservatives, according to the aides, who spoke on the condition of anonymity because they were not authorized to talk publicly about the talks.

Spokesmen for Manchin and Toomey said only that the senators are talking to many of their colleagues about gun legislation and could not confirm details of their discussions.

Toomey is usually a reliable conservative vote for Senate Republicans, but he faces re-election in a Democratic-leaning state in 2016. A new player in the months-long gun talks, he is one of several GOP senators who have said that they would be receptive to supporting an expanded background-check program if a bipartisan deal were to emerge. As a former House lawmaker, Toomey remains close to House Republicans who represent the suburbs of Philadelphia and Pittsburgh, some of whom have said that they are open to striking bipartisan compromises on gun legislation in part because support for new gun laws is strong in those areas of the state.

Manchin, a moderate Democrat with an "A" rating from the National Rifle Association, has been eager to strike a deal on gun-control legislation since the Sandy Hook Elementary School massacre in December that left 20 children and six educators dead in Newtown. He has spent months negotiating with Sens. Charles Schumer, D-N.Y., Mark Kirk, R-Ill., and Tom Coburn, R-Okla., over details, but talks have stalled in recent weeks amid Coburn's opposition to Schumer's insistence on requiring gun owners to keep records of private gun sales. Despite the impasse, Schumer and Coburn have "kept the lines of communications open" in case a deal can be struck, one aide said.

With Coburn's support waning and Kirk's moderate blend of politics not seen as enough to bring along other Republican senators, Manchin has spent much of the two-week congressional recess seeking out other GOP supporters. Talks between Manchin and Toomey began in earnest last Wednesday, and the two have swapped proposed drafts, aides said. Any formal announcement of a deal won't come until Tuesday or Wednesday, when the men return to Washington and sort out remaining details in person, aides said.

If Manchin succeeds in striking a deal, Senate Majority Leader Harry Reid, D-Nev., plans to move quickly to include that language in the bill, according to top aides. Those aides caution that Republicans could force the process to extend into next week by exercising various procedural rules. Reid's current bill includes several of the leading Democratic proposals to curb gun violence, including a plan to expand background checks to all gun purchases, make gun trafficking a federal crime for the first time and bolster federal financing of school security programs.

Schumer said Sunday that Democrats are open to changing the bill's language on background checks if Manchin can strike a deal with Republicans.

"Please let us go to the floor," Schumer said Sunday on CBS News' "Face The Nation." "If we go to the floor, I'm still hopeful that what I call the sweet spot - background checks - can succeed. We're working hard there."

At the White House, top officials remain confident of brokering a deal on universal background checks, which is the most politically palatable of Obama's gun-control proposals. The negotiations largely are being run out of Vice President Joe Biden's office, with his chief of staff, Bruce Reed, playing a leading role. Administration officials privately concede that bans on assault weapons and high-capacity ammunition magazines are unlikely to pass the Senate when Reid holds up-or-down votes on the proposals as amendments to the main gun bill.

In hopes of increasing public support for passage of a gun bill, Obama on Monday will fly to Connecticut - where emotions are still raw more than 100 days after the massacre - to deliver a speech on gun violence at the University of Hartford. A White House official said Obama would "speak, as he did at the State of the Union, of the obligations we have to children lost in Newtown and other victims of gun violence to act on these proposals."

On Tuesday, Biden will hold a gun-control event at the White House with law enforcement leaders, while first lady Michelle Obama will return Wednesday to her home town of Chicago to speak about gun violence from the perspective of a mother. Over the past couple of years, Chicago has experienced a surge in gun violence.

But Republicans also are mounting a campaign to stop new gun legislation. At least 13 Senate Republicans, led by Rand Paul, Ky., Mike Lee, Utah, Ted Cruz, Texas, and Marco Rubio, Fla., are threatening to block any new gun legislation, with aides expecting more GOP senators to sign on to the filibuster threat when they return to Washington this week.

"The Second Amendment to the Constitution protects citizens' right to self-defense," the senators wrote in a letter they plan to send to Reid today. "It speaks to history's lesson that government cannot be in all places at all times, and history's warning about the oppression of a government that tries."

Dan Pfeiffer, a senior Obama adviser, warned Sunday that any filibuster would go against an overwhelming majority of the country.

"What the president wants to sign is a strong, bipartisan bill with enforceable background checks," Pfeiffer said on "Fox News Sunday." "That has 90 percent support. It can get done."

Pfeiffer recalled Obama's State of the Union address in February, in which the emotional high point was his call for a vote on gun-control measures. With families of Newtown victims, as well as former congresswoman Gabrielle Giffords, D-Ariz., who was shot in her Tucson district, in attendance, Republican and Democratic lawmakers stood in applause.

"Now that the cameras are off and the families aren't there, they are engaging in legislative tactics to make this harder," Pfeiffer said of Senate Republicans. "There's no reason we have to do that. And, as the president said, politics is the only reason this stuff won't get done."

Sen. John McCain, R-Ariz., also called on conservatives not to filibuster gun-control measures on the Senate floor because Reid has said he is open to permitting votes on GOP amendments.

Appearing on CBS's "Face the Nation," McCain wouldn't immediately commit to supporting background checks on gun purchases at firearms shows, but said he would look at the specifics in a final proposal. "It really depends on how they are carried out, how long, what the depth of it is. This is another reason why we need to go to the floor," he said.

Former Republican congressman Asa Hutchinson, who led an NRA-commissioned study that endorsed placing armed guards in schools to curb gun violence, also appeared open to the idea of expanding background checks in commercial transactions while exempting private sales between friends and family members.

"If you go to a gun show and you're buying a firearm from a licensed dealer and have a background check, but you also go out to somebody's vehicle and you get a firearm there, and you purchase it and you don't have a check, there is some inconsistency there. And certainly, from my personal standpoint, that's a fair debate," he said on "Fox News Sunday."