WASHINGTON — Rep. Eric Swalwell of California might have left the presidential race, but his signature issue is still on the primary ballot.

After the mass shootings in El Paso and Dayton earlier this month, Swalwell’s proposal for a mandatory buyback of all assault-style weapons was embraced by the congressman who represented the Texas border city, former Rep. Beto O’Rourke — giving the idea its highest-profile platform yet.

Swalwell has been actively advising presidential candidates to follow his lead, telling them in conversations that “the moms will have your back,” even as critics denounce the plan as gun confiscation that would turn ordinary Americans into criminals.

Collected firearms at Riccarton Racecourse on July 13, 2019 in Christchurch, New Zealand, apart of the first firearms collection event to be held following changes to gun laws, providing firearms owners the initial opportunity of many to hand-in prohibited firearms for buy-back and amnesty. Handout / New Zealand Police via Getty Images

“If you want to ban future sales, I actually think intuitively you're already there,” Swalwell said. “You recognize these are weapons we can’t have in our communities, and the next step shouldn't be that hard, which is to get the ones that are there now.”

This week, a group led by survivors of the Parkland shooting unveiled a plan that included the same concept — a departure from more common proposals that would only ban the future sale of affected firearms.

Democratic candidates and prominent gun safety groups, however, are still mostly unwilling to embrace the idea, with some, like former Vice President Joe Biden and Vermont independent Sen. Bernie Sanders, responding with calls for making any buyback programs voluntary. Up until fairly recently, it was hard to find any nationally known politicians arguing that gun owners should be forced to turn in their weapons, period — even as many praised Australia for implementing a similar law.

The party instead focused largely on passing legislation that enjoys bipartisan support in Congress, like universal background checks and "red flag" laws that would take guns from those deemed a danger to themselves or others. The leading background check bill is co-authored by Sen. Joe Manchin, a West Virginia Democrat who, incampaign ads, shoots legislation he doesn’t like.

But this year the conversation is charting new territory.

Frequent mass shootings, continued inaction in Congress and a highly competitive presidential field have contributed to a race to embrace fresh ideas or revive old ones abandoned after opposition from gun owners.

“I think they've drastically changed, both in terms of how candidates are prioritizing the issue versus 2016 or 2012 or 2008, but also in terms of the boldness of those ideas,” said Igor Volsky, founder of Guns Down America Action Fund, which advocates for reducing the total number of firearms in America.

And as the years have passed without a breakthrough in Congress — Trump has entertained background check legislation repeatedly only to seemingly back away — politicians and activists are growing less concerned about whether Republicans will go along with their proposals or the opposition of gun rights groups.

“Clearly what we’ve seen is that approach has failed, and the reason this has failed — and it’s reflected in our politics on every issue — is there’s no interest on the other side to ever do anything,” said Volsky.

In addition to the mandatory buybacks O'Rourke is championing, other 2020 Democratic candidates have sought proposals to make themselves stand out. This week, Massachusetts Sen. Elizabeth Warren called for a threefold and fourfold tax increase on guns and ammunition, respectively. New Jersey Sen. Cory Booker, meanwhile, has made requiring a federal license to buy firearms a centerpiece of his campaign, and others, like O’Rourke, Warren, and South Bend, Indiana, Mayor Pete Buttigieg, have adopted similar plans.

“You have 20 Democratic presidential candidates each trying to look more anti-gun rights than the other for their base, so they have to have different proposals,” Alan Gottlieb, founder of the pro-gun Second Amendment Foundation, said. “It’s more heated. It’s more in-your-face.”

Driving this new push is also a demographic shift within the party. The current Democratic House majority is built on suburban swing seats where the winning candidates often made support for new gun laws a major part of their campaigns. Nearly200 Democrats in the chamber are co-sponsoring a bill to ban assault weapons, which picked up its first Republican backer this week in Rep. Peter King of New York.

At the same time, gun safety activists and experts cautioned that some of the policy proposals being floated by Democrats this year are more well-established ideas than others.

“I wouldn’t put licensing and mandatory buybacks in the same bucket,” Peter Ambler, executive director of Giffords, a gun control group founded after the wounding of then-Arizona Rep. Gabby Giffords and killing of six others in a mass shooting at a constituent meeting in Tucson.

While there's no clear recent precedent for a mandatory buyback program for assault weapons, Ambler said, gun licensing is the norm in many places.

More than a dozen states, including North Carolina and Nebraska, already have various licensing requirements for gun owners, necessitating permits and background checks before certain firearms purchases. Former President Bill Clinton called for a licensing program for handguns in every state in his final State of the Union address, less than a year after the school shooting in Columbine, Colorado.

Such requirements have gained steam in recent years as academics have suggested they might be significantly more effective than simply expanding background checks. A study by Johns Hopkins University researchers found that gun homicides declined by 40 percent and firearm suicides by 15 percent in Connecticut after the state passed a permit law in 1995, while Missouri saw a 25 percent increase in gun homicides and a 16 percent rise in firearm suicides after repealing a similar law in 2007.

“If you want to keep guns from high-risk people, you need to figure out who they are during the sale of the gun, but it looks like licensing is the most effective way to do that,” Jon Vernick, co-director of the Center for Gun Policy and Research at the Johns Hopkins Bloomberg School of Public Health, told NBC News.

Experts have raised concerns about how to implement proposals like a mandatory buybacks that require gun owners sell their assault-style weapons, which could make up as many as 15 million of the more than 300 million privately owned firearms in the U.S., according to a National Rifle Association estimate.

Swalwell says his plan would make it easier to comply by allowing owners to keep their guns if they store them at a shooting range or hunting club.

Warren, along with Sen. Bernie Sanders, has proposed an approach backed by gun safety groups like Giffords that would regulate assault-style weapons in a fashion similar to machine guns, which are rarely associated with crime. Machine guns are no longer allowed to be manufactured, but the law banning them grandfathered in existing ownership so long as those in possession registered their guns and paid a $200 tax.

Ambler, the Giffords executive director, said he can "understand with this epidemic of mass shootings where the desire is coming from" for a mandatory buyback plan. But he said regulating ownership instead "would utilize an existing policy framework" and be simpler to achieve as a result.

But Second Amendment activists have argued many gun owners would likely refuse to comply with bans on existing guns, especially a mandatory buyback or requirements they register their guns, making it impossible to implement without generating new conflict with law enforcement.

Jon Caldara, president of the libertarian Independence Institute, has been protesting a Boulder, Colorado law banning assault-style weapons and requiring owners of existing weapons to show the police proof of prior purchase in order to keep their existing stock. As part of his campaign against the new rules, he's deliberately refusing to comply and predicted gun owners would react similarly to national efforts requiring them to turn in their firearms.

“The euphemism they use for gun confiscation is just delicious,” he said. “What you’re doing is turning good guys into criminals.”