Updated: 11:31 a.m.

A Democratic senator seeking to expand the national gun background check system has found a new Republican partner, likely boosting the prospects for a new bipartisan agreement that senators could vote on in the coming weeks, according to several senior Senate aides familiar with the talks.

Sen. Joe Manchin (D-W.Va.). Sen. Joe Manchin (D-W.Va.).

Sen. Joe Manchin III (D-W. Va.) has been speaking in recent days with Sen. Pat Toomey (R-Pa.) about supporting a plan that would exempt private gun sales between close family members from the background check system and possibly provide other exemptions, said the aides, who were not authorized to speak publicly about the talks.

Sen. Pat Toomey (R-Pa.) Sen. Pat Toomey (R-Pa.).

Toomey is usually a reliable conservative vote for Senate Republicans, but faces reelection in a Democratic-leaning state in 2016. A new direct player in the months-long gun talks, Toomey is one of several GOP senators who have said he would be receptive to supporting an expanded background check program if a bipartisan deal were to emerge.

Toomey's office didn't immediately return requests for comment, but other senior aides involved in ongoing talks confirmed his involvement, adding that they now expect the  formal announcement of a deal on Tuesday or Wednesday once senators return and sort out remaining details in person.

Senate Majority Leader Harry M. Reid (D-Nev.) previously announced plans to begin debate on a gun bill this week that would 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. But Democratic aides say Reid will delay the start of debate until a bipartisan compromise on background checks can be reached, likely pushing the formal start of consideration into the week of April 15.

Nine in 10 Americans support expanding the gun background check system, according to several recent national polls, but supporters of stricter gun laws are concerned that the Senate will be unable to approve such legislation if Reid can't stitch together a coalition of at least 60 senators to defeat a threatened Republican filibuster. Sen. Rand Paul (R-Ky.) and other conservative senators have said they will block any new gun legislation when it reaches the Senate floor.

Dan Pfeiffer, a senior White House adviser, on Sunday expressed optimism lawmakers could pass a measure to expand background checks for gun purchases, and warned Republicans against filibustering.

"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."

Sen. Charles E. Schumer (D-N.Y.) and Sen. John McCain (R-Ariz.) joined Pfeifffer's call for conservative Senate Republicans not to filibuster gun control measures on the Senate floor. A group of senators including Paul, and Sens. Ted Cruz (R-Texas), Mike Lee (R-Utah) and Marco Rubio (R-Fla.) has threatened to filibuster legislation.

"I do not understand why United States senators want to block debate when the leader has said we can have amendments," McCain said on CBS News's "Face The Nation."

The Arizona Republican wouldn't immediately commit to supporting background checks on all firearm purchases at gun 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," McCain said.

Manchin, a moderate Democrat with an "A" rating from the National Rifle Association, has been eager to strike a deal on new gun control legislation ever since a gunman killed 20 schoolchildren and six educators at the Sandy Hook Elementary School in Newtown, Conn. last December. He has spent months negotiating with Scuhmer, Mark Kirk (R-Ill.) and Tom Coburn (R-Okla.) over details of a plan, but talks have stalled in recent weeks amid Coburn's opposition to Schumer's insistence on requiring gun owners to keep records of any private gun sales.

Schumer said he remained optimistic about expanding background checks and is open to modifying the existing legislation on the matter.

"Please let us go to the floor," Schumer said on "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. Senator Manchin and Senator Kirk have a few ideas that might modify the proposal that we put in there. As long as they don't impair the effectiveness, I'm entertaining those."

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.

Pfeiffer said on ABC News's "This Week With George Stephanopoulos" that the White House is still in communication with lawmakers in both parties, including Coburn.

"The White House is still talking to Senate Republicans, including Senator Coburn, and Senate Democrats."

Former Republican congressman Asa Huthchinson, who led a National Rifle Association-commissioned study that endorsed placing armed guards in schools to curb gun violence, argued on "Fox News Sunday" that expanding background checks without exemptions and requiring record-keeping would impose burdensome obstacles upon some gun owners, and wouldn't address school safety concerns.

"I don't know whether that is going to pass or not, but it's not going to address the problem of safety in schools," said Hutchinson, who said he was only expressing his own views, not the NRA's.

But Hutchinson 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.