By Richard Cowan and Doina Chiacu

WASHINGTON (Reuters) - Democrats pushing for gun curbs after the latest mass shooting in the United States are co-opting a Republican mantra to build public support and defang opposition: it's time to get tough on national security.

Shoring up national security has long been a pillar of Republican orthodoxy, as has staunch opposition to gun control.

But the massacre of 49 people in Orlando, Florida, last Sunday, the deadliest mass shooting in modern U.S. history, by a gunman who pledged loyalty to Islamist militants may be leaving Republicans on shakier ground.

With national security driving the debate, Democrats see a more powerful argument than simply advocating the need to curb gun violence in a country of 320 million that has more than 310 million weapons.

Although the Orlando gunman, Omar Mateen, is believed to have had no help from extremist Islamist groups in targeting a gay nightclub, he had been investigated by U.S. authorities for possible links to terrorism and subsequently cleared.

That prompted Democrats to clamor for legislation to expand background checks and prevent people on U.S. terrorism watch lists from buying guns. Votes on four measures were scheduled Monday in the U.S. Senate, two sponsored by Democrats and two by Republicans. Many Republicans, and some Democrats, oppose strict gun curbs partly on constitutional grounds.

"Every senator is now going to have to say, whether they're for terrorists getting guns or against terrorists getting guns," Democrat Senator Chuck Schumer told reporters on Thursday.

"The terrorists that we need to fear are not on the streets of Aleppo, or Mosul or Fallujah. They're on the streets of the United States and they will have guns unless we pass tough laws," added Senator Ed Markey, a Massachusetts Democrat.

President Barack Obama, Democratic presidential candidate Hillary Clinton and Homeland Security Secretary Jeh Johnson all took the tack this week that gun measures were a safeguard against terrorism.

Republicans have long criticized Obama for not being tough enough on national security and doing more in the fight against Islamic State.

The Orlando massacre and the San Bernardino, California shooting in December by a couple inspired by Islamic State captured the attention of the American public in a way previous mass shootings have not, said Tom Diaz, a former member of the National Rifle Association gun rights lobby who now backs gun control.

"They've changed the dynamic of this whole issue,” said Diaz, an author and expert on terrorism and the gun industry.

That shift in sentiment has heartened the families of the 20 elementary school children and six staff members killed in Newtown, Connecticut in 2012, who championed the last big, and ultimately unsuccessful push, on gun control.

About 71 percent of Americans, including eight out of 10 Democrats and nearly six out of 10 Republicans, favor at least moderate regulations and restrictions on guns, according to a Reuters/Ipsos poll conducted from Monday to Thursday. That was up from 60 percent in late 2013 and late 2014.


Diaz believes Republicans must look as if they care about keeping guns out of the hands of so-called homegrown extremists, while balancing issues of due process and the Constitution's Second Amendment right to bear arms that form the backbone of the NRA's opposition to gun control.

Republicans say new laws won't necessarily keep weapons out of the hands of people intent on doing harm, and are keen to avoid twinning the two issues.

"This is not a gun control issue," U.S. Senator Ted Cruz said on Thursday. "This is a terrorism issue."

Republican strategist Ron Bonjean said Democrats "must be careful about overplaying their hand with rhetoric that could sound like government overreach to Americans who believe in the Second Amendment.”

Even if the current efforts fail, the new push on national security may prove Democrats' best shot at eventually luring Republican support on an issue that has floundered for decades.

"This is a chance for the Democrats to talk in tough terms about safety and security and also to link that to the gun issue," said Robert Spitzer, political science professor at State University of New York at Cortland.Some notable Republicans appeared willing this week to engage in the debate on gun control. The party's presumptive presidential nominee, Donald Trump, vowed to meet with the NRA to talk about ways to bar people on certain government watch lists from buying guns.

The top Republican in the Senate, Majority Leader Mitch McConnell, said he was open to suggestions from experts on how to prevent terrorism suspects from acquiring firearms and called the Orlando shooting a "calculated act of terror."

But it was unclear whether Trump or McConnell would throw their weight behind any measures acceptable to Democrats.

Democratic U.S. Representative Jim Himes said he did not hold out great hope that the gun legislation would advance.

"The reason you won't see a compromise anytime soon is because Congress actually acting in the wake of Orlando would be a tacit admission on the other side that guns had something to do with what happened in Orlando as opposed to ISIS," he said, using an acronym for Islamic State.

(Editing by Mary Milliken)