Convening in southern Nevada for a labor union's annual forum this weekend, Democratic presidential candidates soon found themselves reacting to news of "another mass shooting."

"One is speechless in trying to make a rational statement about another mass shooting in this country," Sen. Bernie Sanders told reporters at the AFSCME forum in Las Vegas.

"It is deeply concerning that we have Senate leadership more concerned with pleasing the NRA, than listening to the vast majority of the American people," the Vermont independent lamented. Senate Minority Leader Chuck Schumer has called on the majority leader, Sen. Mitch McConnell to bring the Senate back for an emergency session on guns.

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Saturday's shooting in El Paso, followed by another massacre overnight in Ohio, thrust frustration over congressional inaction back into the spotlight at the gathering of 19 White House hopefuls, mere minutes from the Las Vegas Strip where a gunman killed nearly 60 in 2017 in the bloodiest mass shooting of modern American history.

"This is beyond anything we should be tolerating," said former Vice President Joe Biden on Saturday, after news broke of the attack in Texas. "We can beat the NRA. We can beat the gun manufacturers," he predicted.

Bills advanced by the Democratic-led House this year to expand background check requirements for gun purchases have stalled in the GOP-led Senate, despite widespread public support.

Beyond background checks, candidates have coalesced around a range of measures to tighten America's gun laws, including so-called "red flag laws," opening gun manufacturers and dealers to liability, allowing federal funds to research gun violence, and banning assault weapons, where public support has waned.

"If my recollection serves correctly, a majority of Americans said they didn't want seatbelts in cars," Sen. Kamala Harris said Saturday. "Certain things just must be done for the best interest of the whole and this is one of them."

The alleged El Paso shooter was arrested Saturday after gunning down 20 and leaving dozens more injured at a Walmart in El Paso. The Dayton gunman was killed by police, after his attack in a popular local nightlife district.

Each wielded an assault weapon in their rampages.

News of the massacre in El Paso prompted former Texas Congressman Beto O'Rourke to abruptly cancel planned campaign events in Nevada and California, returning home to the Texas border town he used to represent.

"I just ask for everyone's strength for El Paso right now. Everyone's resolve to make sure that this does not continue to happen in this country," O'Rourke told reporters before departing the Las Vegas event, visibly shaken.

"This is really about hatred and racism and intolerance that continues to grow in this country," he later told "Face the Nation" from El Paso, accusing the president of sowing division.

O'Rourke, like several of the Democratic presidential candidates, laid blame for the shootings at the president's feet.

"I'm saying that President Trump has a lot to do with what happened in El Paso yesterday," O'Rourke added.

Former Obama housing chief Julián Castro made a similar point, the fellow Texan telling ABC: "Anybody who has the ability to see and hear and understand what the president has been doing since he started his campaign in 2015 knows that division and bigotry and fanning the flames of hate has been his political strategy."

"When the president speaks, his or her words have incredible consequence," Harris told reporters Sunday, before a church service in Las Vegas. "We have a president of the United States who has chosen to use his words in a way that have been about sowing hate and division."

Sanders sounded a similar note hours later at a high school in Las Vegas, pleading President Trump to "please stop the racist, anti-immigrant rhetoric. Stop the hatred in this country which is creating the kind of violence that we see."

"We need to call out white nationalism for what it is — domestic terrorism," Sen. Elizabeth Warren tweeted on Sunday, calling out "the president himself for advancing racism and white supremacy."

President Trump said Sunday in response to the shootings, "Hate has no place in our country. And we're gonna take care of it." He also said he had spoken with Attorney General Bill Barr, FBI Director Christopher Wray, the governors of both Texas and Ohio and members of Congress and would be working with authorities in the coming days.

Mr. Trump attributed the shootings in part to mental health issues. "This has been going for years. For years and years and we have to get it stopped," Mr. Trump said. "Perhaps more has to be done. This is also a mental illness problem. These are people who are very, very seriously mentally ill."

Prosecutors are pursuing hate crime and domestic terror charges against the gunman in El Paso. Among the evidence is an anti-immigrant manifesto, which may have been penned by the suspect.

Last week another gunman had posted a recommendation for a book popular among white supremacist groups, before his deadly attack at a Gilroy food festival in California.

"Certainly the El Paso incident, based on everything I've read so far suggests that yes — this is an act of domestic terrorism," Harris said, accusing the Trump administration of failing to "call these crimes what they are."

"America is under attack by a lethal, violent, white nationalist terrorism. And if we are serious about confronting it, that means we have to have a different conversation than this handwringing," Pete Buttigieg said, as reports emerged of the El Paso manifesto.

The South Bend mayor, who served in Afghanistan as part of a counter-terrorism unit, declared the situation a "national security emergency."

"Enough is enough," Sen. Cory Booker told CBS News on Saturday, after remarking at a church service in Las Vegas about the shooting. "We need to protect our nation from these heinous terrorist attacks," the New Jersey Democrat added.

Eleanor Watson contributed to this report