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Here are the candidates who have put out proposals so far, and what we know about the candidates who have yet to do so. The list doesn't include every Democratic candidate, but highlights candidates who have significant positions or experience that sets them apart -- whether they switched positions over the years, held office at the time a mass shooting occurred or have experience with guns through military service.
What could gun legislation look like if a Democrat wins the presidency?
So far, Sen. Cory Booker, D-N.J., has carved out one of the most ambitious lanes in the gun policy debate ahead of 2020 -- a plan he calls "the most comprehensive gun violence prevention plan of any candidate for president in decades."
Booker's recently announced policy would require all gun owners to acquire a license through the federal government. Currently, 16 states have similar laws to varying degrees.
Former Obama administration Housing and Urban Development Secretary Julian Castro also supports gun buybacks, which he acknowledged "have had mixed success," but said are "good policy" in some circumstances. Castro, who is from Texas, also supports universal background checks, limiting the capacity of magazines and an assault weapons ban. He highlighted the importance of mental health and research in a recent CNN town hall.
"I would think about this in a kaleidoscopic way. It's not only about the issue of guns themselves, although I do believe that we need common sense gun reform. It's also about things like mental health that touch on what leads people to use guns in a bad way," Castro said.
Sen. Kamala Harris, D-Calif., has also announced her intentions to act on gun control, which she said she would enact within her first 100 days through executive action.
Harris would require near-universal background checks to be run by people selling more than five guns in a year and revoke the licenses of gun manufacturers and dealers that break the law. She has also talked about renewing a 1994 law that banned assault weapons but expired in 2004.
Other senators vying for the presidency -- including Sens. Amy Klobuchar of Minnesota, Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts -- have yet to roll out formal 2020 policies, but have years of congressional records that shed light on their positions.
For Klobuchar, judging gun control legislation is about what would "hurt [her] Uncle Dick in the deer stand," a perspective that comes from her state's value of the outdoors, including hunting and fishing, Klobuchar said in a CNN town hall in February.
"And so I come at it from a little different place than some of my colleagues running for this office," she said.
But those factors don't keep her from supporting universal background checks and a ban on assault weapons, she said. In the past, Klobuchar has applied the same rule about her Uncle Dick to other laws, like preventing people on a terrorist watch list from buying guns.
"Would closing off the loophole in the terrorist watch list hurt my Uncle Dick in his deer stand? Not at all," Klobuchar said in 2016.
Much like Klobuchar, Sanders has many constituents who hunt. According to Vermont's Department of Fish and Wildlife, there are 66,000 residents who hunt and the state rakes in nearly $4 million in hunting license revenue each year.
In the past, the rural connection has led to criticism by some progressives that Sanders was too moderate on gun control, especially in his early career. The Vermont senator has made a point since kicking off his 2020 campaign to show he's evolved.
During the 2016 battle for the Democratic election, Sanders frequently was attacked for repeatedly voting against a 1993 law that established federal background checks. At the time, Sanders told a local newspaper that he supported background checks but was opposed to the federally mandated waiting period that came along with it because it might affect local gun shop sales.
Ahead of 2020, Sanders often brings up his lengthy record to prove a different point: He has consistently voted since the 1980s to ban semi-automatic assault weapons.
I'm running for president because we must end the epidemic of gun violence in this country. We need to take on the NRA, expand background checks, end the gun show loophole and ban the sale and distribution of assault weapons.— Bernie Sanders (@BernieSanders) February 20, 2019
Warren, born and raised in Oklahoma, has said that she values the rights of "law-abiding citizens" to own guns, though she's a strong advocate for gun law reform and has voted for an assault weapons ban. After the deadly shooting in Parkland, Florida, in 2017, Warren wrote letters to several major companies that invested with gun manufacturers and asked them to pressure the industry to change.
"I encourage you to take action to ensure that the gun companies in which you invest are taking steps to reduce gun violence," Warren wrote to Fidelity, BlackRock, Vanguard Group and others.
In an interview in February, Warren said gun violence would be qualified as a national emergency "right off the top" if she were elected.
An issue that affects candidates and voters nationwide
Only one of the nearly two dozen major Democratic presidential candidates has not seen a mass shooting take place in their state while in office: Rep. Tulsi Gabbard of Hawaii.
For the purpose of this analysis, ABC News defined a mass shooting as an incident where four or more people were shot or killed, not including the shooter.
Harris and Swalwell, for example, who have both made gun control a key part of their 2020 platforms, held office in California during two of the state's most deadly mass shootings in recent history: when 14 people were killed in San Bernardino in 2015 and when 12 people were killed in a shooting at a college bar in Thousand Oaks in 2018.
After a shooting at an Aurora, Colorado, movie theater in 2012, killed 12, Hickenlooper signed gun control measures that required background checks for private and online gun sales and banned high-capacity magazines. Bennet also voted to ban high-capacity magazines, but in 2010 voted to oppose restricting the right to bear arms and in 2009 voted to allow Amtrak riders to check bags containing guns.
During former Vice President Joe Biden's nearly five decades in public office, there were dozens of mass shootings, which has in turn made Biden a longtime advocate for gun control. As a U.S. senator, he introduced the assault weapons ban, which was signed into law in 1994.
And after the deaths of 20 first grade students and six staff members at Sandy Hook Elementary School in 2012, then-Vice President Biden led a new task force on gun violence. Four months after the tragedy in Connecticut, Biden's modest measure on background checks passed the House, but didn't make it out of the Senate.
Who has switched their stance on guns?
Sometimes candidates change positions.
For former Texas Rep. Beto O'Rourke, a minor switch came just a few months into the 2020 cycle, after Booker announced his proposed plan.
Asked about establishing federal gun licensing, O'Rourke initially told reporters on the campaign trail that it went "too far." But after a night of reflection, he changed his mind.
"What I think Senator Booker has done is to push us past that to seek to do even more, and I'm grateful that he's done that and I, you know, as I've thought about it, I really think that we should be looking at everything," O'Rourke said the next day on the trail.
For other candidates, like the former governor of Colorado, the evolution on guns has been a longer, more complicated process.
"This wasn't a Colorado problem. This is a human problem," Hickenlooper told ABC News' George Stephanopoulos in 2012, in the wake of the Aurora shooting. "Even if he didn't have access to guns, this guy was diabolical. … He would have done something to create this horror."
Since then, Hickenlooper has adapted his tone, and more than once. Colorado has passed measures expanding background checks and limiting magazine sizes to 15 rounds, despite backlash from gun rights groups.
Sen. Kirsten Gillibrand has also evolved, tumbling from an "A" rating from the National Rifle Association to an "F" -- a point of pride, she's said.
"I didn't have much of an understanding about what gun violence actually looks like in a community," Gillibrand said in a 2016 podcast about her past pro-gun stance, when she represented a rural upstate New York district.
She was changed by conversations with families who'd been devastated by gun violence in cities, she said.
Gillibrand is joined by Rep. Tim Ryan of Ohio, who was also formerly given an "A" rating from the NRA. Ryan shifted his stance more recently than Gillibrand, speaking out after the Sandy Hook Elementary School shooting in 2012. He has since donated roughly $20,000 to gun control organizations.
How veterans who were trained to use guns view them on U.S. streets
There are three candidates in the race who were trained to use guns in combat. Even though their legislative records differ, all three support a ban on military-style assault rifles.
After 49 people were killed at Pulse, a nightclub in Orlando, Florida, in 2016, Marine veteran and current Rep. Seth Moulton tweeted a photo of himself in uniform holding a military-grade rifle saying, "I know assault rifles. I carried one in Iraq. They have no place on America's streets."
The rifle the Massachusetts congressman was holding is not available to civilians and was not the same gun that was used in the nightclub shooting.
Moulton also authored bipartisan legislation to ban bump stocks, an accessory that increases the firing rate, after the device was used in the deadly mass shooting in Las Vegas that killed 58 people in 2017.
"There are common sense reforms, there are common sense laws that we can pass, that are respectful of gun rights but still will reduce this public health crisis in America," he said in an interview on ABC's "This Week" with co-anchor Martha Raddatz after the Las Vegas shooting.
Also after the shooting in Las Vegas, South Bend, Indiana, Mayor Pete Buttigieg, a U.S. Navy Reserve veteran who was deployed to Afghanistan, similarly denounced AR-style rifles.
"I did not carry an assault weapon around a foreign country so I could come home and see them used to massacre my countrymen," he tweeted.
Gabbard, a major in the Hawaii Army National Guard, has come under criticism for not sponsoring gun control measures that are widely supported by other Democrats in the House. But she has voted to ban assault weapons, and, shortly after the deadly high school shooting in Parkland, Florida, killed 17, Gabbard added her name as a co-sponsor of the Assault Weapons Ban of 2018.