Hours after a gunman killed 10 people in a grocery store in Boulder, Colo., the second deadly shooting in less than a week, gun-control activists once again called for lawmakers to take action.

But even as they did so, some acknowledged that progress on the issue has been frustratingly slow. Across the country, lawmakers at all levels have struggled to pass meaningful legislation, and courts sometimes strike down what does become law.

It's an even bigger challenge at the federal level, where no major gun-control measure has passed in more than two decades, even though nearly 20,000 Americans were killed by guns last year, according to the Gun Violence Archive.

"Solutions are not the priority for our politicians. It's never ending," said Manuel Oliver, whose son Joaquin, 17, was killed in the mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla., in 2018.

"I know what those families are going through exactly and it makes me feel so sad," he said. "And it makes me feel that we are not doing enough."

Another Parkland activist, Cameron Kasky, echoed that frustration.

"There's no amount of cruelty that will cause substantial gun control in this country," Kasky, 20, said in an interview. "Kids can be killed and killed and killed, and at best you will see nothing more than the baseline legislation."

After Monday's King Soopers shooting, President Joe Biden signaled that he wanted to change the dynamic, vowing not to wait "another minute" to propose a ban on assault weapons and high-capacity ammunition magazines, as well as an expansion of background checks.

But at a Thursday news conference, Biden said gun control was a longer-term goal, after the coronavirus recovery.

Shannon Watts, founder of Moms Demand Action and known for marshaling thousands of moms wearing red T-shirts for hearings on gun bills, pushed back on the notion that gun control advocates haven't made progress.

"Saying nothing has changed is not true and it's actually feeding into the gun groups and the NRA narrative that we can't make change," she said.

Watts, along with a coalition of groups, successfully pushed for laws that require background checks on handgun sales in 22 states and the District of Columbia, and red-flag laws in 19 states, which allow courts to seize firearms from those who may harm themselves or others.

Experts agree that the movement is more well-funded and organized now than it has been in the past decade, although the gun rights lobby still vastly outspends it.

But, she said, federal lawmakers need to take action.

"Everyone's been waiting - 25 years - for Congress to act, for that cathartic moment," Watts said. "We're playing the long game."

The group just launched the social media campaign #MoreThanThoughtsAndPrayers to spotlight the hollow condolences of politicians and called on Biden to issue executive orders and work to pass background checks for gun purchasers at the federal level.

Meanwhile, gun rights advocates are girding for a major battle, with organizations rallying their membership to fight any proposed legislation.

The National Rifle Association remains powerful despite charges of lavish spending by its officials and the fact that the lobbying group announced it was filing for bankruptcy protection amid a legal effort in New York to dissolve it.

NRA spokesman Amy Hunter Wright said the group has continued to work to pass "pro-Second Amendment legislation" throughout the United States, including open carry bills in Utah and Montana as well as bills that ensure firearms retailers won't be closed in a state of emergency.

Mike Hammond, legislative counsel for Gun Owners of America, said the Boulder shooting showed gun control doesn't work, noting that Colorado already has universal background checks, a red-flag law and a large-capacity magazine ban.

"With everything we have, we'll fight to keep meaningless, counterproductive gun control from passing," Hammond said.

Alan Gottlieb, the founder of the Second Amendment Foundation, said his group was readying for the Democratic-led Congress and the Biden administration to push for "as much as they can get" on gun control.

"We're geared up for a big battle," he said.

His group's membership has grown since Biden took office, adding 70,000 to its ranks of about 700,000. The organization is running a national TV advertising campaign to push back against gun control.

The political landscape around guns has changed drastically in the past decade, experts said, after the school shooting in Newtown, Conn., in which 26 people died, including 20 children between 6 and 7 years old. The tragedy helped spur the emergence of a well-organized, well-funded gun-control movement that is still outspent by gun rights groups - some $168 million to $24 million for lobbying over the past two decades, according to a study from the Center for Responsive Politics's OpenSecrets.org.

"Newtown was definitely a moment that shook the country and shook up the gun debate," said Adam Winkler, a UCLA School of Law professor who specializes in constitutional law and the Second Amendment. "It didn't shake it up enough to get actual federal legislation on the issue, but it did shake it up enough to reinvigorate the gun-control movement."

Before Newtown, he said, Democrats rarely made guns a major focus in campaigns, afraid that doing so would cost the party in swing states. Meanwhile, the vast majority of advocacy around guns focused on protecting gun rights, with little organized opposition.

At the state level, change has been widespread since Newtown - on both sides.

"In the bluer states, the response was, 'Oh my God, we need to do more to regulate and restrict gun access,' " said Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research. "And in red states, it was precisely the opposite."

Polling has found a bipartisan majority of Americans support regulations such as background checks, which has been a focus on the gun-control side. Meanwhile, gun rights groups pushed to expand the number of places where guns can be carried.

Red-flag laws have emerged as one of the only measures that can win support on both sides, although they are opposed by some gun rights groups that cite due process concerns.

After Parkland, with pressure mounting, Republican-led states including Florida, Maryland and Vermont enacted such legislation.

Gun rights lobbyists are continuing to work on adding more states to the 15 states that don't require a permit to carry a gun in public - including Iowa, which passed a new rollback just days ago that is awaiting Republican Gov. Kim Reynolds's signature.

During the past two election cycles, young survivors of the Parkland shooting became teenage folk heroes of the movement and used their voices, art and networks to grab adult attention with powerful slogans like "We Call BS." They also held one of the largest marches on Washington.

Max Markham, policy director of March for Our Lives, a group that formed after the Parkland shooting, said that the group was instrumental in getting gun-control candidates elected in 2018 and 2020 and in campaigning for new gun laws in Virginia.

"There's still a feeling of optimism, but also a feeling that the clock is ticking," Markham said. "We are what, Day 60 of Biden's first 100 days? We need to get their attention now."

Gregory Jackson, a national organizing director of the Community Justice Action Fund and gunshot wound survivor, said lawmakers should pay attention to the root causes of gun violence, which remains pervasive in many underserved communities. He points to a shooting in Chicago on March 14 in which three people died that got scant attention.

"Too often we pivot to the gun - the hardware - and that's one big part," he said. "I'm talking about focusing in investing in neighborhoods most impacted by gun violence, which is all just a side effect of neglect. There's lack of housing stability, educational resources, mental health services. I'm talking about quality of life and safe spaces that disappeared during the pandemic and we saw gun violence rise."

He's hoping for a boost in funding for programs in cities that will invest in food and job support for the families and survivors so they can recover, as well as in conflict resolution classes and free mental health therapy for those most at risk for violence. His group trains community-based activists to work against policies such as "Stand your ground rules," which have been used against Black and Brown communities.

Asked about the potential for change at the federal level, Jackson sighed and thought for a moment. "Well, we hope it's just not lip service and thoughts and prayers."

Longtime Boulder gun-control activist Dawn Reinfeld, who co-founded the gun-control group Blue Rising, called for a statewide ban on assault weapons in the wake of this week's attack.

In the coming days, Reinfeld's organization said the group will be working to get the Boulder assault weapons ban reinstated, planning a virtual town hall on Wednesday. The city banned the possession, transfer and sale of most shotguns and certain pistols and semiautomatic rifles in 2018, a move almost immediately challenged in court.

"I would say I feel discouraged," she said. "We're grieving, but at the same time I think there are more and more people who are saying, 'I'm not going to take this anymore.' "

- - -

The Washington Post's Jennifer Jenkins and Tim Craig contributed to this report.