(CNN) As the Democratic presidential candidates fanned out across early contest states Monday, the traditional Labor Day kickoff to the fall campaign was overshadowed by their frustration about the congressional deadlock over gun safety laws after yet another deadly mass shooting over the weekend.

While mingling with voters in Iowa and New Hampshire Monday, the top tier candidates were continually peppered with questions about how they would stop gun violence following the latest massacre in Odessa, Texas , which killed seven people. As candidates were shaking hands and gearing up for the next round of campaigning, authorities revealed that the West Texas gunman who went on a shooting spree in Odessa after being fired from his job at Journey Oil Field Service, had previously failed a gun purchase background check.

Former Texas Congressman Beto O'Rourke spent part of Monday at a victims' event in Odessa.

"Unfortunately, we share this bond now, In the wake of this massacre, this tragedy," said O'Rourke, whose hometown of El Paso, Texas, was the site of another mass shooting last month. "But I know that we also share a fierce pride in who we are, a determination not to be defined by this act of terror, or violence, or hatred, but instead by the way in which we will overcome this and define ourselves to the rest of the country."

He spoke of keeping guns out of "the hands of those who should not have them" and taking military-style weapons off the streets.

"We can have that conversation that we can take the action that should necessarily follow not as Democrats, not as members of unions, not as Republicans, but as Americans first, as human beings," he said.

Former Vice President Joe Biden called out President Donald Trump and Republican leaders for their failure to move gun legislation, calling the intransigence in Washington "disgraceful."

"The idea that we don't have elimination of assault type weapons -- magazines that can hold multiple bullets in them -- it's absolutely mindless," Biden said in Cedar Rapids, Iowa, Monday before attending the Hawkeye Area Labor Council Picnic. "It is no violation of the Second Amendment -- it's just a bow to the special interest of the gun manufacturers and the NRA. It's got to stop."

When reminded by a reporter that his campaign is premised on his unique ability to broker compromise with Republican leaders like Senate Majority Leader Mitch McConnell, Biden said gun control is one area where he sees little hope for a deal.

"I'll work with Mitch McConnell where we can agree, but on this one, he's not going to agree, because he is where the President is," the former vice president said. "So we just have to beat them, flat out beat them. ... I think there's no compromise. This is one we have to just push, push, push, and push, and push. And the fact of the matter is, I think it's going to result in seeing some of them defeated."

At a house party in Hampton Falls, New Hampshire, a woman told Massachusetts Sen. Elizabeth Warren that it seems as though mass shootings are happening on a weekly basis now: "It's a big deal for a day and then everybody forgets about it," she said.

"The overwhelming majority of Americans want to see us take sensible gun safety regulations, they want to see us change the laws," Warren replied. "We know what the pieces are. ... Universal background checks, assault weapons off our streets, get rid of bump stocks, and the ability to fire weapons in a short period of time. There are a lot of things we could be doing. So, why doesn't it happen?"

"The answer is corruption. It's corruption. Right now, we have a Washington that is being held hostage by the gun industry and the NRA. That has to stop."

Campaigning in Maine and New Hampshire, Vermont Sen. Bernie Sanders also accused Trump and McConnell of cowering when faced with the power of the NRA.

"Donald Trump, Mitch McConnell and all of these guys are scared to death of the NRA," Sanders said Sunday night during his campaign's first events in the caucus state of Maine. "So I say to McConnell... and to Trump, I say to them: Listen to the American people, not just to the NRA. The NRA can no longer dictate gun policy in this country.... The House passed some decent legislation, (I) think we've got to go further, but McConnell has got to have the guts to put that legislation on the floor of the Senate."

After Trump was seen out golfing on the Labor Day Holiday as Hurricane Dorian hurdled toward the East Coast, California Sen. Kamala Harris said she was mystified by the President's seeming indifference to the families affected by the Odessa tragedy, as well as the millions of Americans under hurricane evacuation orders, and the families of some 34 people who were missing after a dive boat caught fire Sunday night off the coast of California.

"He just does not have the ability to really have a sense of empathy for people who are enduring hardship and enduring pain," Harris told reporters while attending a Labor Day rally for workers in Los Angeles. "The fact that, as you said, he is playing golf while this is happening is not surprising. It's not surprising, but it certainly is not reflective of the leadership that a president should provide in these moments of great tragedy."

In Iowa City, South Bend, Indiana, Mayor Pete Buttigieg also chided the president for golfing as Dorian barreled toward Florida: "Every time we think there's a new low, we see it get lower."

"As I recall, the president said he was remaining in the US to deal with the emergency," Buttigieg said. "Apparently, his idea of dealing with the emergency is to go golfing. To be fair, when he shows up in emergency zones, I'm not sure it helps or hurts, but it would be nice for him to at least show some respect for the Americans who are currently fleeing for their lives from evacuation zones."

Earlier in the day at a climate change roundtable in Iowa, Buttigieg told reporters he hoped. Congress could reach a middle ground compromise to reduce the number of mass shootings: "A very middle of the road, conservative compromise would be to do background checks and red flag laws, and a perhaps a milder version of an assault weapons ban."

"That's the compromise, because that's the middle of what the American people want," Buttigieg said. "Otherwise we're bargaining against ourselves."

He added that Republicans who oppose those sorts of measures should face a "politician penalty."

Getting ready for fall

While the gun control debate was at the forefront over the Labor Day holiday, the presidential candidates were also doing their best to gear up their get-out-the-vote operations behind the scenes for the fall campaign.

Buttigieg, who has raised a great deal of money but has so far retained middling polling numbers, announced slates of new staffers and new offices in Iowa and New Hampshire while fending off questions that he was playing catch-up to more established candidates like Warren and Sanders.

"We're reaching the point where we will be literally 100 times as many people as we were the day that we began," Buttigieg said in Cedar Rapids. "You want to make sure you do that in stages."

And indeed, it has started to show: The campaign said Monday that it was adding 36 organizers in Iowa -- bringing their paid staff to about 100 in the Hawkeye state -- and that it would open 20 field offices in the 20 days leading up to the Polk County Steak Fry on September 21. It also announced a New Hampshire expansion, with 12 new office openings and by hiring 16 additional paid organizers, bringing his total number of staff in the Granite State to 56 people.

While campaigning in New Hampshire, Sanders and Warren fired the most significant shots yet in what is likely to end up as a pivotal battle among the progressive favorites in the first-in-the-nation primary.

Each demanded a fairer economy, affordable health care, measures to combat climate change and fundamental reforms to purge Washington politics of the controlling influence of big money.

But both also rejected an emerging storyline -- and the implication of the Biden campaign -- that they are too radical to woo the political center and beat Trump.

Warren was asked one of the key questions of the election by Linda Rhodes of Indivisible New Hampshire, a progressive group -- whether she can beat Trump in November 2020.

"I know how to fight and I know how to win and that is a big part of it," answered Warren, who was making her 15th trip to the state since January. She pushed back against the idea that Democrats should spike their most progressive goals as part of an electability strategy.

"I think what is going to carry us Democrats is not playing it safe," she said, warning that the party's nominee should not be "nibbling around" structural problems in US society.

Sanders is effectively defending home turf in the Granite State, where his thumping win over Hillary Clinton in 2016 was an early sign of lukewarm grass roots enthusiasm for her campaign.

But Warren, whose campaign has had a strong summer has, like Sanders strong roots and a media profile in a neighboring state to New Hampshire. Their duel in the battleground could help decide which of them emerges as the pre-eminent progressive challenger in the long primary campaign.

Sanders held a string of events and rallies over the long weekend in the state and was in an exuberant mood as he marched in the Labor Day parade in the town of Milford.

"Every day gets a little bit closer to Election Day," Sanders said in a nod to Labor Day's fabled role as the kickoff to the fall campaign.

Sanders is also going out of his way to counter perceptions that he is too liberal to beat Trump. As the sun set on Labor Day Eve, he rallied a crowd of supporters in the former mill town of Dover, New Hampshire, where workers facing draconian conditions in the early 19th century staged the first strike by women in US history.

Sanders portrayed his previous campaign and the subsequent move left by the Democrats as proof that his ideology is not so extreme that it would alienate moderate voters.

Touting his ideas for single payer healthcare and a rise in the minimum wage, Sanders told his New Hampshire supporters: "What you showed the country is that the ideas that we are talking about that the establishment said were too far reaching, too radical, you said, 'This is exactly what this country needs.'"

"Thank you, New Hampshire."