WASHINGTON >> As Congress wrestles with how to respond to a wave of mass shootings, leading Democrats are raising an idea once viewed as political suicide: reviving the ban on assault weapons, which barred Americans from purchasing certain military-style firearms for a decade until Republicans let it expire in 2004.

The idea is gaining traction on the presidential campaign trail, where former Vice President Joe Biden, an architect of the original 1994 assault weapons ban, and nearly all of the other Democratic candidates have embraced it. In an opinion piece published today in the New York Times, Biden vowed to make the 1994 law “even stronger,” adding, “We have to get these weapons of war off our streets.”

Two centrist Democrats who flipped Republican House seats last year — Reps. Mikie Sherrill of New Jersey and Jason Crow of Colorado — also came out on Monday in favor of the ban, with an opinion piece in USA Today. Both are military veterans; Crow ousted a Republican incumbent after running on an aggressive platform of combating gun violence.

With strong opposition from Republicans, who are in charge of the Senate, and President Donald Trump in the White House, an assault weapons ban has virtually no chance of being signed into law before 2021. Nearly 200 House Democrats are backing legislation to reinstate the ban, which is not enough to even pass the House, and voting on such a measure would be politically risky for vulnerable moderates.

Still, the push by prominent Democrats — including former President Bill Clinton, who signed the original ban into law and outlined his support for it last week in an essay in Time magazine — does force the issue onto the 2020 campaign agenda. And it demonstrates just how much the politics of gun safety have changed over the last several years — and especially the last few weeks, after the back-to-back massacres in El Paso, Texas, and Dayton, Ohio.

“The public has supported the assault weapons ban and they really support it when you remind them that we had it already and that these are weapons that the military uses,” said Celinda Lake, a Democratic pollster. “The intensity has always been on the anti-gun-control side. Now the intensity is shifting onto the other side and the refrain out of Dayton — which is exactly the refrain the public has — is, ‘Do something.’”

Polls show that a majority of Americans support an assault weapons ban, but the support is not bipartisan. A July poll by National Public Radio found that 57% of respondents were in favor of a ban on the sale of semi-automatic assault guns such as the AK-47 or the AR-15. But while 83% of Democrats said they were in favor of the ban, just 29% of Republicans supported it.

For years, the assault weapons ban was considered politically toxic for Democrats. After Clinton signed the ban into law in 1994, Democrats were trounced in the midterm elections. They lost control of the House, which they had held for 40 years, and among those who lost their seats was Speaker Tom Foley, who drew the ire of the National Rifle Association when he came out in favor of the ban and was the first sitting speaker to lose an election since 1862.

The outcome rattled Democrats, and their fears of the NRA only grew after Al Gore lost the presidency to George W. Bush in 2000. Many Democrats, including Clinton, blamed the loss on Gore’s stance on gun control.

“The NRA could rightly claim to have made Gingrich the House speaker,” Clinton wrote in his autobiography in 2004, referring to Newt Gingrich, the Republican who succeeded Foley.

That same year, the assault weapons ban expired. Some Democrats remain skittish about it.

“There’s still a lingering worry, but I think it’s dissipating a bit,” said Jim Kessler, vice president for policy at Third Way, a centrist Democratic organization, adding, “For probably two dozen House Democrats, it is a risky vote.”

Support for universal background checks — an idea embraced by Trump and by Democrats and some Republicans — is much stronger. The NPR poll found that 89% of Americans support background checks on all gun purchases, including sales at gun shows and on the internet. A bill mandating background checks has already passed the House, and Sen. Mitch McConnell, the Senate Republican leader, has said he expects the Senate to debate the issue when it returns in September.