Here are five GOP lawmakers who have taken issue with the ways their party's presidential candidate has reacted to the massacre in Orlando. (Sarah Parnass/The Washington Post)

Presumptive Republican presidential nominee Donald Trump and some vulnerable GOP lawmakers signaled Wednesday that they are open to changing the nation’s gun laws, raising the possibility that the political tide might be shifting on an issue that has sharply divided Americans for years.

This isn’t the first time a mass shooting, in this case the massacre in Orlando that killed 49 people this week, has sparked a national cry for congressional action on gun availability. In past instances, such calls to action have been followed by intense pressure campaigns from gun rights supporters urging lawmakers to focus elsewhere.

But the debate launched this week could be different, mostly because of Trump. And it could further divide Republicans, many of whom were already distancing themselves from the presumptive nominee’s anti-Muslim rhetoric in the wake of Orlando.

Trump’s renewed focus on gun laws goes against GOP orthodoxy, which generally considers Second Amendment issues to be settled. It also complicates the Republican response to the Orlando shootings, which had focused mostly on national security and concerns about home-grown terrorism.

And Trump’s attention to the issue has the potential to strain lawmakers in tough reelection battles: those from moderate swing states with constituents who favor tougher gun laws; and those who feel pressure to defend against Trump’s critique that nothing gets done in Washington.

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On Wednesday, Trump said he would schedule a meeting soon with the National Rifle Association to discuss proposals to ban people on certain federal watch lists from buying firearms. Trump was renewing a position he first expressed last year after a mass shooting in San Bernardino, Calif. But on Wednesday he took it to a new level, via Twitter, by calling for a meeting with the NRA.

Three Republican senators took similar stands. Sen. Rob Portman (R-Ohio), who is locked in a closely watched reelection battle, told Ohio reporters that he is ready to back a federal ban on weapons sales to anyone on a terrorist watch list if a compromise can be reached. Sen. Susan Collins (R-Maine) said much the same. And Sen. Pat Toomey (R-Pa.), who has worked on bipartisan gun-control legislation in the past and is also facing a tough reelection challenge, announced plans late Wednesday to introduce new legislation after a day of talks with members of both parties as well as gun-control groups.

After the San Bernardino shooting in December, Trump told CBS’s “Face the Nation” that he is “very strongly into the whole thing with the Second Amendment. But if you can’t fly, and if you have got some really bad — I would certainly look at that very hard.”

“We have to be looking at a lot of different things,” Trump added in the interview. “But we can’t do anything to hurt the Second Amendment. People need their weapons to protect themselves. And you see that now more than ever before.”

Trump’s position was buoyed this week by Senate Majority Leader Mitch McConnell (R-Ky.), who said that he is “open” to new laws to keep guns out of the hands of terrorists but not through existing proposals authored mostly by Democrats. Republicans have objected to those proposals because of the risk, they say, that innocent Americans improperly listed might be denied the right to purchase a weapon. Their counter-proposals have included provisions that would require the FBI to prove that someone has engaged in terrorist activity before banning a gun purchase.

“Nobody wants terrorists to have firearms,” McConnell told reporters on Tuesday.

Sen. Pat Toomey (R-Pa.) is in talks with a leading gun-control group about banning terrorism suspects from purchasing firearms. (Larry Downing/Reuters)

The NRA expressed general support for the idea — but it also made clear that a ban on sales to all people on watch lists would be unfair. Only after an investigation by the FBI proving terrorist activity should such sales be blocked, the group said. “Our position is no guns for terrorists — period. Due process & right to self-defense for law-abiding Americans,” the group tweeted.

More ardent gun rights activists warned that Trump and GOP lawmakers could be jeopardizing conservative support by pushing for changes.

“This is the first true character test for Donald Trump, undoubtedly. As of right now, it sure looks like he’s failed it miserably,” said Dudley Brown, president of the National Association for Gun Rights, an NRA rival.

Brown said it should “scare every American” that federal officials could place them on “a secret list that would block their constitutional rights.” He will be attending the Republican National Convention in Cleveland next month as a delegate and said that he and others are hoping to rewrite the party platform with tougher gun rights language.

“But we’re not waiting for Cleveland,” Brown said, noting that NAGR members were already calling and emailing GOP lawmakers in Washington to forestall a fresh debate.

Meanwhile, Democrats on both sides of Congress were launching a full-court press aimed at forcing Republicans to act.

On the Senate floor, Sen. Chris Murphy (Conn.) launched a filibuster with other Democrats amid debate on a Justice Department spending bill that they want to amend with gun-control proposals.

“I am prepared to stand on the Senate floor and talk about the need to prevent gun violence for as long as I can,” Murphy tweeted at the start of the filibuster.

House Democrats also said Wednesday that they would push again to expand background checks on gun purchases and renew the lapsed ban on assault weapons — but would focus first on stopping terrorism suspects from buying firearms.

The federal terrorism watch list included some 800,000 people as of September 2014, the most recent data available. The Orlando shooter, Omar Mateen, was on a list until 2014, when the FBI removed him on the grounds that he did not pose a credible threat. The lead shooter in the San Bernardino massacre also had been investigated by the FBI.

That revelation sparked a quick but fruitless attempt to revamp gun laws last year. Democrats introduced legislation by Sen. Dianne Feinstein (Calif.) that would have granted the attorney general power to prevent suspected terrorists from buying weapons, irrespective of whether their names appear on the government’s official lists. Republicans responded with a bill by Sen. John Cornyn (Tex.) that would have given authorities three days after a suspected terrorist tried to purchase a gun to prove there was probable cause to reject the sale.

Neither plan passed. Only two Democrats voted for Cornyn’s plan, with most believing that federal authorities could never clear a potential suspect within three days. Collins supported Cornyn’s bill at the time, but on Wednesday she said it did not go far enough.

Cornyn and Feinstein said Wednesday they were in talks about a potential compromise — but Feinstein said later in the day that those talks had broken down.

Sen. Mark Kirk (Ill.), another embattled incumbent up for reelection, was the only Republican to back Feinstein’s bill in December. But other GOP senators voted against the measure, citing the concerns of gun rights advocates including Richard Feldman, head of the Independent Firearm Owners Association. He worried that everyday Americans mistakenly placed on government watch lists might struggle for years to clear their names and restore their Second Amendment rights.

“Once in America, we used to be considered not guilty until proven guilty. Now we’re guilty until proven innocent. And it’s very hard to prove innocence,” Feldman said Wednesday.

Given that divide, Toomey’s discussions with former New York mayor Michael Bloomberg’s group, Everytown for Gun Safety, were seen as a promising development for gun-control advocates and members of both parties eager for a compromise.

“This is not rocket science to figure this out,” Toomey said on the Senate floor Wednesday, adding that he had been speaking to “several” senators about a new bill. He didn’t mention any talks with Bloomberg’s group, but spokesmen for both sides confirmed that the group was in touch with the senator.

In 2013, Toomey partnered with Sen. Joe Manchin (D-W.Va.) in the wake of the 2012 Newtown, Conn., school shooting in a push to expand criminal and mental-health background checks at gun shows and for online sales. Their proposal earned 54 votes — six shy of the 60 needed to advance. Only four Republicans supported the bill.

Portman, who voted against Toomey’s bill in 2013 and Feinstein’s proposal in December, told reporters Tuesday: “I hope that the entire Senate votes to say that if you’re on the terrorist watch list — not just the no-fly list, which is a much more targeted list, but the terrorist watch list — you should not be able to buy a weapon.”

Other lawmakers also took steps Wednesday to revive the debate.

Sen. Bill Nelson (D-Fla.), who was on the scene of the Orlando shooting shortly after it occurred, proposed requiring the FBI to be alerted if a terrorism suspect buys a gun. The alert would not bar the suspect from purchasing the weapon but would notify the agency and allow it to take action, he said.

Other Democratic senators introduced a plan to funnel $190 million to counterterrorism efforts and active-shooter training at the FBI. The bill would also create 36 new positions to track terrorist threats.

Feldman, the gun rights advocate, speculated that real legislative action is not likely.

“The problem is that when the emotionalism of this moment fades, as it will — it may be two weeks or two months, but it always does fade — the people who care about this issue are the gun owners of this country,” Feldman said. “They will vote for or against people on the basis of this issue.”

Mike DeBonis, Catherine Ho and David Weigel contributed to this report.