2020 candidates convene on gun safety forum on the heels of Las Vegas shooting anniversary

With the nation still reeling from a spate of mass shootings over the summer, a new season now kicks into gear - and with it, a painful anniversary: two years since the Las Vegas massacre - the deadliest mass shooting in modern U.S. history.

On the heels of that grim milestone, nine 2020 Democratic candidates on the campaign trail are convening in Las Vegas Wednesday for a forum on gun control - the first of its kind for presidential hopefuls – co-hosted by a nonprofit founded by former Rep. Gabrielle Giffords and March for Our Lives, in partnership with MSNBC.

Sen. Bernie Sanders, D-Vermont., was also set to attend, but had to abruptly cancel campaign events after being hospitalized for an artery blockage.

The surge of gun violence episodes like that in El Paso, Texas, and Dayton, Ohio, have placed the issue front and center on the 2020 stage. Democratic primary candidates have responded with renewed emphasis on gun control proposals, and a push for more progressive plans across the board.

Former Rep. Beto O’Rourke, D-Texas, has been increasingly outspoken since the mass shooting in his own hometown of El Paso, Texas, killed 22 people -- becoming the first candidate to call for a mandatory government buyback of assault weapons. He is also the only candidate signed onto the March for Our Lives Peace Plan, which outlines an ambitious gun reform agenda.

The March for Our Lives’ plan proposes the creation of a new federal position -- a director of gun violence prevention -- who would report directly to the president.

It also calls for changing the standards for gun ownership, halving the rate of gun deaths in a decade, strengthening accountability for the gun lobby and industry and generating new community-based solutions.

Sen. Elizabeth Warren, D-Mass., announced an extensive proposal in the wake of El Paso, blending executive actions and legislation to expand background checks, close loopholes, enforce an assault weapons ban and institute a licensing system -- much like the Peace Plan. However, while March for Our Lives calls for buybacks, Warren does not.

March for Our Lives, a movement that grew out of the Parkland shooting in February 2018, wants to halve the rate of gun deaths in one decade; Warren calls for an 80% reduction in gun deaths, but had not been specific about how long that would take - until Wednesday - for the first time on stage, providing an important time frame for her gun plan.

Questioned at the forum by a young student, Briana Smith, Warren didn't blink.

"Over eight years!" she said. "That's known as two terms for a president!”

It's a more ambitious goal than the March for Our Lives Peace Plan.

Sen. Cory Booker, D-N.J., would require gun owners to acquire a license through the federal government -- something which O’Rourke considered "too far" before evolving his position.

O’Rourke has retooled his message with a sharp central focus on gun control; it has breathed fresh life into a campaign that had previously fizzled.

Ahead of the gun forum, O’Rourke called on his fellow candidates to sign onto March for Our Lives’ plan. No one besides O'Rourke has signed on.

March for Our Lives confirmed that all of the candidates invited to the forum were made aware of the plan, and have acknowledged receipt of it.

"The goal being set is to encourage everyone to come out with a comprehensive approach that’s been lacking in the past," Eve Levenson, one of March for Our Lives’ national leaders, told ABC News. "While we’d obviously love to have more support for the peace plan, it’s more important to have a comprehensive national approach, whatever you're going to call the plan."

Levenson said she and the movement have not heard specific reasons from any of the candidates why they have yet to endorse the plan. ABC News has reached out for comment from the campaigns for their view.

Levenson wants candidates not just to say gun violence is a top priority, but to demonstrate "bold, holistic plans" that address all aspects of gun violence.

"I think any candidate running for office should be prepared to follow through on what they promise," Levenson said. "We want to see this is a top issue for them, and that they look at this with an intersectional lens -- not just focused on mass shootings, but look at gun violence as a whole, as a public health crisis, all root causes."

ABC News' Cheyenne Haslett contributed to this report.