Some of the gun lobby’s strongest allies are breaking with the National Rifle Association to support proposals that would expand background checks for private firearm sales.

In behind-the-scenes talks with congressional staff members and others, gunmakers, dealers and other Second Amendment advocates have offered support for more instant criminal background checks, buoying the hopes of gun-control supporters, including President Obama, who has put a top priority on extending criminal checks to private sales.

The trade group for the nation’s leading firearm manufacturers said it will not actively oppose the expansion of background checks, which are designed to prevent guns from reaching criminals or the seriously mentally ill.

“That’s more the NRA’s issue,” Steve Sanetti, president of the National Shooting Sports Foundation (NSSF), said in an interview. “From the commercial side, we’re already there, and we’ve been there, and we were the ones that have been the strongest proponents of an effective, complete background check.”

Sensing an opportunity, a group of big-city mayors wrote to major gun manufacturers Monday, warning that their governments may begin using economic leverage to win support for increased regulation, including an extension of background checks. The mayors of Los Angeles, Minneapolis, Philadelphia and other large cities spend millions on firearms and ammunition each year for their police departments.

The Washington Post’s Ed O’Keefe shows us some of the guns targeted by legislation that would outlaw certain rifles, pistols and high-capacity magazines. (The Washington Post)

“Our residents want to ensure that the tax dollars that are being used to purchase guns for our police departments are going to manufacturers that share our values and support our strategies,” Minneapolis Mayor R.T. Rybak, who leads the National Conference of Democratic Mayors, wrote in a letter to Glock and the NSSF.

The recipients did not comment on Tuesday.

The effort to drive a wedge between the gun industry and the NRA intensified ahead of a Senate Judiciary Committee hearing Thursday on the Democrats’ gun-control program. The plan includes proposals for an assault-weapons ban, new restrictions on gun trafficking, school safety and criminal background checks.

Gun manufacturers have been wary of crossing the NRA, in part because of the bruising experience of Smith & Wesson more than a decade ago. During the Clinton administration, the firearm maker privately negotiated gun-safety issues. Gun rights groups swiftly punished Smith & Wesson, urging dealers not to carry the company’s products and creating serious financial trouble.

Representatives from some gunmakers are discussing details of the expanded background check proposal in confidential talks with congressional staff, according to people familiar with the talks who spoke on the condition of anonymity because of the sensitivity of the conversations.

Gun store owners and retail dealers, for the most part, have been more vocal in supporting an expansion of background checks after the shootings at Sandy Hook Elementary School in Newtown, Conn., in December. Because commercial establishments are already required to perform them, an expanded background check requirement probably would increase traffic and business for licensed dealers.

Rep. Mike Thompson (D-Calif.), a hunter and Vietnam veteran leading a gun task force in the House, said that every dealer he talks to lately expresses support for more background checks. He says the NRA no longer represents the views of most gun owners.

In a statement Monday, NRA spokesman Andrew Arulanandam said that expanding background checks is “a wrongheaded approach.”

He said that “the NRA is supportive of background checks on retail sales to ensure criminals and the mentally ill with violent tendencies do not have access to firearms.”

“Unfortunately,” he added, the National Instant Criminal Background Check System “is currently incomplete and has inaccurate data. Rather than focusing on improving the quality of information contained in NICS, gun control proponents are advocating a significant expansion of a system that has gaping inadequacies.”

Late last month, in a speech in Salt Lake City, NRA Executive Vice President Wayne LaPierre criticized the background check proposal, saying it is “aimed at one thing: registering your guns. And when another tragic opportunity presents itself, that registry will be used to confiscate your guns.”

LaPierre openly supported expanded checks a decade ago.

Not all of the NRA’s traditional allies share this sense of alarm. For example, the founder of the pro-gun Second Amendment Foundation tentatively backed a proposed compromise bill in Washington state last month that would expand checks while limiting state firearms record-keeping.

In addition, the head of the nation’s largest police union, which was allied with the NRA in a major legislative battle in the past, has joined the movement for expanded background checks.

Jim Pasco, executive director of the Fraternal Order of Police, said in an interview that he now supports extending checks to gun shows and other venues where they are not required.

“Closing that gap would make it much more difficult for criminals to obtain firearms,” he said.

Sens. Tom Coburn (R-Okla.), Charles E. Schumer (D-N.Y.), Joe Manchin III (D-W.Va.) and Mark Kirk (R-Ill.) have been meeting for weeks to try to reach consensus on a plan to expand criminal background checks for private sales.

Talks faltered last week when Coburn said he would oppose provisions mandating extensive recordkeeping on gun sales. He subsequently received repeated calls from Obama and Vice President Biden. Coburn said Tuesday that the senators are “very close” to a deal, adding, “I think we’ll eventually get there.”

Gun-control advocates say the group, which includes two NRA-backed senators, already has accomplished a lot: breaking with the NRA and agreeing in general on the idea of expanded background checks.

Proponents of expanded checks point out that a records system for gun purchases is already in place. When a person buys a firearm from one of the country’s 60,000 federally licensed dealers, the purchaser is required to fill out paperwork that stays with the dealer until the store is closed down. Only then are the records sent to the Bureau of Alcohol, Tobacco, Firearms and Explosives. The records can help trace firearms that are used in crimes.

Federal law prohibits the establishment of a national gun registry. But advocates of expanded checks say some recordkeeping is necessary because federal authorities would otherwise be unable to trace guns used in crimes.

Figuring out how to meet law enforcement needs while ensuring that the recordkeeping does not constitute a government-backed database is one question the four senators are contemplating.

More gun rights advocates are expressing support for the expansion. “I support instant background checks on the purchase of all guns to prevent convicted felons from obtaining them,” said Sen. Johnny Isakson (R-Ga.), who met recently with the families of shooting victims in his state.

Some well-known gun rights advocates are joining with bipartisan lawmakers to support expanded background checks at the state level.

In Washington state last month, the head of a gun rights group offered to support mandatory background-check legislation for most firearm sales in exchange for a state commitment not to maintain gun records. It’s not clear whether the proposal will succeed but it has drawn support across the divide of the gun debate.

“This is a good compromise with real give-and-take,” said Alan Gottlieb, founder of the Second Amendment Foundation and chairman of the Citizens Committee for the Right to Keep and Bear Arms.

Joel Achenbach and Ed O’Keefe contributed to this report.