The long-simmering debate in this country over gun rights took a dramatic turn Thursday when the National Rifle Association unexpectedly joined an effort to restrict a device used to accelerate gunfire in the Las Vegas massacre.

The NRA’s announcement gave political cover to a growing number of Republicans who have indicated a willingness to consider regulating “bump stocks,” devices that allow a legal semiautomatic rifle to mimic the rapid discharge of a fully automatic weapon. Less clear is whether the move signals an opening for further action on an issue that has divided the nation and produced virtually no new restrictions in recent years despite a steady stream of mass shootings.

“The NRA believes that devices designed to allow semi-automatic rifles to function like fully-automatic rifles should be subject to additional regulations,” read a statement issued by the powerful organization Thursday.

Federal law enforcement officials have said that Las Vegas gunman Stephen Paddock fired weapons outfitted with bump stocks Sunday, leaving 58 dead and hundreds injured in a matter of minutes. Experts have said that audio of the attack makes clear that the shooter unleashed a torrent of bullets faster than he could have fired without adapting his rifles.

As the country’s largest gun rights group, the NRA exerts considerable influence among conservative voters who support the organization — and on the GOP’s approach to gun policy. Many Republicans have operated under the fear that opposing NRA positions could lead to primary challenges. But public opinion is also on the minds of Republicans as they head into a midterm election year that is expected to be contentious. Regulating bump stocks could help the party combat perceptions that it has done nothing to address the mass shootings.

Will Congress ban bump stocks, a gun accessory used in the Las Vegas attack?

The sheer carnage of Sunday’s attack is fueling lawmakers’ interest in the issue, said Sen. John McCain (R-Ariz.). “Look at Las Vegas. That’s how I account for it,” McCain told reporters. “Americans are horrified by it. They’re horrified, and they should be.”

Still, even after the group’s announcement Thursday, only a handful of Republicans had stepped forward to consider examining bump stocks.

House Speaker Paul D. Ryan (R-Wis.), House Majority Leader Kevin McCarthy (R-Calif.) and House Judiciary Committee Chairman Bob Goodlatte (R-Va.) all said Thursday that lawmakers will consider further restrictions on the devices. More than a dozen Senate Republicans said they were open to the possibility. A few of Congress’s most conservative lawmakers — as well as some of its most avid supporters of gun rights — said the restrictions were worth consideration.

“I didn’t know what a bump stock was until this week,” Ryan said at a news conference in Chestertown, Md. “A lot of us are coming up to speed. . . . Having said that, fully automatic weapons have been outlawed for many, many years. This seems to be a way of going around that, so obviously we need to look how we can tighten up the compliance with this law so that fully automatic weapons are banned.”

White House press secretary Sarah Huckabee Sanders welcomed the NRA’s position and said President Trump wants to be part of a “conversation” about cracking down on bump stocks. “We’re open to having that conversation,” Sanders said during Thursday’s White House press briefing. “We think we should have that conversation, and we want to be part of it moving forward.”

Senate Majority Leader Mitch McConnell (R-Ky.) stuck out as one of the only members of Republican congressional leadership who had not indicated he was on board. He told reporters Tuesday that it is “completely inappropriate to politicize an event like this” and declined to answer further questions on the subject.

The NRA’s position Thursday reflected an about-face on a long-standing position of opposing most gun restrictions, a position founded on the philosophy of the “slippery slope” — that allowing such legislation would beget still more, until law-abiding gun owners were deprived of their Second Amendment right to bear arms.

On Thursday, the NRA blamed the Obama administration for authorizing the sale of bump stocks in 2010, based in part on the manufacturer's claim that the device was intended to assist people with "limited mobility" in their hands. At the time, the Bureau of Alcohol, Tobacco, Firearms and Explosives concluded that the bump stock "has no automatically functioning mechanical parts or springs and performs no automatic mechanical function when installed," according to a letter from the bureau that the manufacturer, Slide Fire Solutions, posted to its website. "Accordingly, we find that the 'bump-stock' is a firearm part and is not regulated as a firearm under Gun Control Act or the National Firearms Act."

In the joint statement from the NRA’s executive vice president and chief executive, Wayne LaPierre, and Chris W. Cox, executive director of the NRA Institute for Legislative Action, the group called on ATF to again review “whether these devices comply with federal law.”

Advocates for greater regulations on guns questioned the sincerity of the NRA and Republican leaders, given their unwillingness to support more-substantial restrictions such as an assault-weapon ban.

“The gun lobby has for years boosted devices that effectively convert rifles into machine guns and boasted that you can get away with guns that mimic fully automatic fire,” said John Feinblatt, president of Everytown for Gun Safety, the gun-control advocacy group founded by former New York City mayor Michael Bloomberg. “So it’s hardly a surprise that they’re calling for a review of bump stocks by a friendly regulatory agency rather than legislation from Congress.”

The organization also does not have long-standing connections to the companies that make bump stocks, which do not have much history of lobbying. A recent search of contributions to federal political campaigns, for instance, turned up none from Slide Fire.

And few lawmakers knew what a bump stock was before this week. Talk of the device has taken over Capitol Hill since the shooting, the worst in modern American history. At least a dozen of the 23 firearms recovered from Paddock’s hotel room were modified to include the accessories, which can be purchased online for a few hundred dollars.

Some lawmakers turned to YouTube to watch videos showing how the devices work.

“That’s what I did yesterday,” said Sen. Susan Collins (R-Maine). “I don’t think most people in the Senate were familiar with this.”

Support for a possible ban has started to coalesce around several pieces of legislation. One measure, unveiled Wednesday by Sen. Dianne Feinstein (D-Calif.), would ban the sale, transfer and manufacture of bump stocks, trigger cranks and other accessories that can accelerate a semiautomatic rifle’s rate of fire.

Feinstein’s bill had support from 38 Democrats as of Thursday morning, including Sens. Bill Nelson (Fla.) and Claire McCaskill (Mo.), who both face uphill fights for reelection next year in conservative states. “The notion that we’re allowing an add-on that allows people to convert a semiautomatic weapon to an automatic weapon — we’ve got to address that,” McCaskill said.

In the House, a bill from Rep. Carlos Curbelo (R-Fla.) would focus on bump stocks but leave out restrictions on other gun accessories. Curbelo said he had been “flooded” with requests from Republicans who want to sign on to the measure, which he planned to introduce Friday. “I think we are on the verge of a breakthrough where when it comes to sensible gun policy,” said Curbelo, a moderate Republican who represents a Miami-area district.

Rep. David N. Cicilline (D-R.I.) introduced legislation similar to Feinstein’s in the House. It had attracted 140 sponsors as of Wednesday night.

Democrats’ electoral map might complicate the debate. Ten Democratic senators, including McCaskill, face reelection bids in mostly rural states that Trump easily won in the 2016 election.

Sen. Heidi Heitkamp (D-N.D.), another such Democrat, said in a statement that she did not know much about bump stocks, “and I first want to learn more about them.”

Sen. Joe Manchin III (D-W.Va.) said that Feinstein’s idea “sounds sensible and reasonable to me,” but he planned to consult hunters in his state before taking a position.

Sen. Joe Donnelly (D-Ind.) issued a statement: “This is a critical and timely issue. I am very concerned about bump stocks, and I am closely reviewing recently proposed legislation.”

In a sign of the far-reaching interest in the issue, even Sen. James M. Inhofe (R-Okla.), an ardent conservative, suggested he is open to supporting the bill. “Not yet,” he said. “I think I probably will eventually.”

On Thursday, a pair of lawmakers began an effort that could preempt legislation on bump stocks. Two House Republicans with military backgrounds, Reps. Mike Gallagher (Wis.) and Adam Kinzinger (Ill.), were gathering signatures for a bipartisan letter asking ATF to revisit its 2010 administrative determination that bump stocks are legal. A group of Democrats made the same request in their own letter to ATF.

Former ATF assistant director Michael Bouchard said in an interview that bump stocks serve “no purpose other than someone to have it and say, ‘This is cool.’ ”

“It serves no purpose for anything,” said Bouchard, who ran the agency’s regulatory and criminal field operations. “Not for sporting, not for target practice, not for hunting.”

David Chipman, a former longtime ATF official who now works as a senior adviser to the gun-control group Americans for Responsible Solutions, said firearm technology has “outpaced the law.”

“Our legislators move at a crawl, and our technology is moving at warp speed,” he said.

Aaron Davis, Sean Sullivan, Sari Horwitz and Paul Kane contributed to this report.