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 Obama and his top surrogates will begin Monday 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.

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.

Private, person-to-person gun sales are relatively common in Maine, a state with a large number of gun owners and a rich hunting heritage but low gun-violence rates. The popular Maine-based trader or swap magazine Uncle Henry’s, for instance, often contains long lists of shotguns, handguns and firearm accessories for private sale, all without background check requirements.

Law enforcement officials and prosecutors in states such as Massachusetts — which has some of the strictest gun laws in the nation — have complained that guns sold on the private market in Maine often end up being used in crimes in the Bay State. A bill co-sponsored by Maine Republican Sen. Susan Collins also up for consideration in the Senate would stiffen federal penalties for gun trafficking and on “straw purchasers” who buy a gun for a prohibited person.

On the issue of background checks, both Collins and independent Sen. Angus King have said they would support expanding the current requirement to private sales. Now background checks are required for any guns sold by a licensed dealer. But both King and Collins say they want to see exemptions for transactions between family members.

Collins has also expressed concerns about how and where the background check records would be maintained.

Toomey is usually a reliable conservative vote for Senate Republicans, but he faces reelection in a Democratic-leaning state in 2016.

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, Conn.

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.

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. Administration officials privately concede that bans on assault weapons and high-capacity ammunition magazines are unlikely to pass the Senate.

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.

On Tuesday, Vice President Joe 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 hometown 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 Senate Majority Leader Harry Reid on Monday. “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 U.S. Rep. 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.

Portland Press Herald Washington Bureau Chief Kevin Miller contributed to this report.