Here's where the 2020 Democrats stand on gun control

Younger voters have put the pressure on Democrats to deliver solutions.

Democratic presidential candidates are both renewing their gun control proposals and pushing for more progressive plans in the wake of recent mass shootings in Odessa, Texas, El Paso and Dayton, Ohio, that left more than 30 victims dead.

Here are the current and former 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 experiences that set them apart -- whether they were the first to support a policy, 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?

Former candidate Sen. Cory Booker, D-N.J., carved out one of the most ambitious paths in the gun policy debate ahead of 2020 -- a plan he called "the most comprehensive gun violence prevention plan of any candidate for president in decades."

Booker's policy, announced in May, would have required all gun owners to acquire a license through the federal government. Currently, 16 states have similar laws to varying degrees.

His plan ultimately pushed federal licensing into the conversation for Democrats, prompting Sen. Elizabeth Warren, D-Mass., former Obama administration Housing and Urban Development Secretary Julian Castro, South Bend, Indiana, Mayor Pete Buttigieg, and former Texas Rep. Beto O'Rourke to echo their support for a national gun licensing program or include it in their own policy announcements.

O'Rourke, who also ultimately ended his bid, demanded banks and credit card companies to stop processing assault weapons sales and firearm transactions without a background check. O’Rourke’s plan was to drum up public pressure by calling on these financial institutions to deny services to all gun and ammunition manufacturers that produce or sell assault weapons. This call appears to be the first of its kind among the Democratic candidates.

Some have also issued support for buyback programs, including former Vice President Joe Biden and Sen. Bernie Sanders, I-Vt. Biden, who authored the 1994 assault weapons ban, is in favor of universal background checks and renewing a ban on assault weapons. However, he has yet to release a detailed plan on gun control reform.

Another candidate who is no longer in the race, Castro, who unveiled a gun policy after the El Paso shooting, also came out in support of buybacks. In the past, Castro has described buybacks as having "had mixed success," but as being "good policy" in some circumstances.

Warren, who did not include support for buybacks in her gun policy announced in the wake of the El Paso and Dayton shootings, described her focus on reducing the power of the NRA, putting $100 million annually toward gun safety research and reducing gun deaths in the country by 80%.

"Historically, when Congress works to address big national issues, we don't simply pass one law and cross our fingers. Instead, we continue the research -- into new policies and around the consequences of our existing policies -- and then come back on a regular basis to update the law," she wrote in her plan. "We don't do this with guns."

"This ends when I'm president," Warren said.

Warren, who was born and raised in Oklahoma and has said that she values the rights of "law-abiding citizens" to own guns, was one of the last front-runner candidates to unveil a gun policy, despite her efforts to be out in front of a host of other 2020 issues by dominating the arena of plans.

Before announcing her policy, Warren had gained support of gun control advocates for her work in the Senate. After the deadly 2017 shooting in Parkland, Florida, 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 her presidential plan, Warren echoed Sen. Kamala Harris, D-Calif., in pledging to act on gun control within her first 100 days through executive action. Harris, too, has since ended her presidential bid.

Harris, who announced her plan in April, said she would also 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 the 1994 law that banned assault weapons but expired in 2004 -- an idea with near universal support among Democratic candidates.

For Sen. Amy Klobuchar, D-Minn., 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.

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.

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, D-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, for example, who has made gun control a key part of her 2020 platform, held office in California when two of the state's most deadly mass shootings in recent history took place: 14 people were killed in San Bernardino in 2015 and 12 people were killed in a shooting at a college bar in Thousand Oaks in 2018.

Colorado has also seen some of the most notable mass shootings in the country and there were two candidates in the race from the state: Sen. Michael Bennet and former Gov. John Hickenlooper. Hickenlooper dropped out of the presidential race on Aug. 15 and later announced that he would run for a Colorado Senate seat in 2020.

After a shooting at an Aurora, Colorado, movie theater in 2012 claimed 12 lives, 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 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?

For O'Rourke, a minor switch came just a few months into the 2020 cycle, after Booker announced his proposed plan, and again after the shooting in El Paso shattered his hometown.

Asked about establishing federal gun licensing in May, 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 Sen. 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.

After the shooting in El Paso, O'Rourke also came out in support of a buyback program.

For the former governor of Colorado, the evolution on guns was a longer, more-complicated process.

"This wasn't a Colorado problem. This is a human problem," Hickenlooper told ABC News Chief Anchor 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, who dropped out of the presidential race on Aug. 28, 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 was joined by fellow former presidential hopeful 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 US streets

There were 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, who dropped out of the presidential race on Aug. 23, 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, 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.