WASHINGTON — President Joe Biden on Tuesday called for tightening of the nation's gun laws, plunging him into an impassioned debate that he largely tiptoed around until it erupted anew after two mass shootings.

But Biden and Democratic leaders tempered their push for swift action with some doubt about their ability to enact new restrictions, even with party control of the White House and Congress, underlining the political volatility that has long surrounded efforts to overhaul gun laws.

In hastily arranged remarks less than 24 hours after a shooting rampage in Boulder, Colo., that left 10 people dead, Biden proposed a ban on assault weapons and high-capacity ammunition magazines, as well as an expansion of background checks during gun sales. Gun-control advocates have tried to push through all these initiatives over the past decade, but strong cultural and political divisions have stymied their efforts.

"I don't need to wait another minute, let alone an hour, to take common-sense steps that will save lives in the future and to urge my colleagues in the House and Senate to act," Biden said. He suggested that he might offer new legislation to complement bills that have already passed the House.

Any gun legislation is expected to face major hurdles in the Senate, which is evenly divided between Democrats and Republicans. Asked during a trip to Ohio later Tuesday whether he had the political capital to shepherd a gun measure, Biden crossed his fingers and replied, "I hope so. I don't know. I haven't done any counting yet."

Still, his comments were the most detailed of his presidency on gun control, an issue that has been eclipsed by the coronavirus pandemic and other crises. Biden engaged in a flurry of executive actions after he was sworn in, but none of them touched on firearms.

And he did not fulfill a campaign promise to send a bill to Congress on his first day in office repealing liability protections for gun manufacturers and closing background-check loopholes.

But the grocery store shooting in Boulder — coming less than a week after a shooting rampage in Atlanta left eight people dead, including six Asian women — has quickly increased pressure on Biden to use the power of his office to secure new gun laws.

"Now is the time to act — not next week, not next month, but today," said John Feinblatt, president of Everytown for Gun Safety. "It's time for the Senate and the administration to do something about it."

Feinblatt, who said he is in touch with the White House, also urged Biden to use his executive powers. "I think it's past time for them to act," he added.

For weeks, the White House has been privately exploring various executive orders related to firearms, such as strengthening background checks and community anti-violence funding, according to people familiar with the conversations.

White House officials confirmed on Tuesday that they are considering potential executive actions, but they declined to provide a timeline.

Also under discussion is regulating "ghost guns," which are devices assembled at home and lacking serial numbers, making them more difficult to track. The people describing the talks spoke on the condition of anonymity to discuss plans that were not public.

The White House focused heavily Tuesday on promoting legislation that has already passed the House to toughen background checks. And administration officials rejected the idea that they have not focused on gun control, pointing to meetings they have had with advocates.

"We are certainly considering a range of levers, including working through legislation, including executive actions to address not just gun safety measures, but violence in communities," White House press secretary Jen Psaki told reporters traveling on Air Force One. "So that is — has been under discussion and will continue to be under discussion."

Along with a surge of immigrants at the U.S.-Mexico border, the shootings have jolted a highly choreographed opening to the Biden presidency, forcing the president to confront polarizing issues he trod over lightly as a candidate. Biden during the campaign portrayed himself as a unifier, emphasizing noncontroversial topics such as pandemic relief and job creation.

Few issues in recent decades have been as contentious as guns. Republicans have used the topic to galvanize their base, warning that Democrats are trying to take away firearms. The attacks have caused Democrats to navigate the issue warily, worried they would pay a political price especially among rural voters if they championed gun-control measures too aggressively.

Biden had a firsthand view of this reality as vice president under President Barack Obama, who made an unsuccessful attempt in 2013 to pass new legislation after the Newtown, Conn., school shooting that left six adults and 20 children dead in December 2012.

Obama on Tuesday also urged the passage of gun-control measures. "We can overcome opposition by cowardly politicians and the pressure of a gun lobby that opposes any limit on the availability of anyone to assemble an arsenal," he said. "We can, and we must." But a statement by Sen. Ted Cruz, R-Texas, at a Senate committee hearing on gun violence made it clear that Republicans are not about to reconsider. "Every time there's a shooting, we play this ridiculous theater where this committee gets together and proposes a bunch of laws that would do nothing to stop these murders," Cruz said.

After a February 2018 shooting at a high school in Parkland, Fla., students launched a wave of activism that propelled gun-control issues to the front of the Democratic agenda, including Biden's. But since taking office, the president has been swamped by other crises, from the pandemic to the economy to immigration.

Meanwhile, there is little evidence that Congress has become any more hospitable to gun control. While there are fewer Democrats representing rural areas, there are fewer moderate Republicans, leading to even greater polarization.

Still, gun-control activists say there are some reasons for hope. They cite the enduring popularity of expanding background checks; they point to the internal problems that have roiled the National Rifle Association, their most powerful opponent; they reflect on the success Democrats have had running on gun issues in the suburban areas where their congressional majorities were built; and they point to a recently successful push to place new restrictions on guns in Virginia.

But those were afterthoughts Tuesday on Capitol Hill. While Democrats spoke of an aggressive push to bring gun legislation to the Senate floor, they carefully calibrated expectations to make clear that the likely outcome was simply a debate and perhaps a vote on some version of a bill introduced previously by Sens. Joe Manchin III, D-W.Va., and Patrick Toomey, R-Pa.

The bills passed by the House are aimed at closing loopholes in the requirement that gun buyers undergo criminal background checks. The "Charleston loophole," for example, allows someone to buy a gun if a background check is not concluded after three days, and that would be extended to 10.

The second bill would close the "gun show loophole," which allows private individuals who are not licensed dealers to buy and sell guns without background checks.

Manchin opposes the two House bills as too far reaching, effectively dooming their prospects in the 50-50 Senate. "No, I don't support what the House passed — no, not at all," he told reporters.

Senate Majority Leader Chuck Schumer, D-N.Y., reminded reporters of an unsuccessful attempt in 2019 to push gun legislation after two massacres just hours apart in Dayton, Ohio, and El Paso, Texas, and said: "This Democratic Senate will be different."

But when pressed, Schumer declined to promise that any particular legislation would pass.

With a 60-vote threshold on legislation in the Senate because of the filibuster, Democrats would need to not only consolidate all of their own supporters, but persuade 10 Republicans to come along.

Sen. Chris Murphy, D-Conn., a chief backer of gun-control legislation since the school massacre in his state, said that he and Schumer plan to sit down soon to map out a strategy.

"I'm a bit loath to compromise on a bill that is so wildly popular, but I obviously would love as big a vote as we can get and for the time being we need 60 votes," Murphy said.

Toomey, who is retiring at the end of the current Congress, was blunt about the challenges to passing gun-control measures.

"We're having preliminary conversations and I hope we can get something across the goal line. But it's very difficult," Toomey told reporters in the Capitol.

Eight years ago, Toomey had three other Senate Republican allies on his efforts — Susan Collins of Maine, who reiterated her support Tuesday for the legislation, Mark Kirk of Illinois, who lost his seat in the 2016 elections, and John McCain of Arizona, who died in 2018. Of the Republicans who have joined the Senate since, none have indicated centrist views on gun control.

In recent years, Republican candidates have touted their guns in campaign commercials. Far-right Rep. Lauren Boebert of Colorado stoked controversy this year with a digital ad in which she vowed to carry a handgun in Washington.

While Democrats face challenges building support for their gun agenda, the opposition also faces a moment of reckoning. The renewed calls for banning assault weapons will test the power of the once seemingly unstoppable NRA. The group has been weakened in recent years, plagued by infighting, allegations of self-dealing among top leaders and a sweeping probe of alleged violations of its nonprofit status by the New York State attorney general.

"Regrettably, gun-control advocates have already rushed to politicize this horrific situation — even as most of the salient facts remain unknown," the NRA said Tuesday.

For Biden, the moment is the latest in a long series of legislative battles over guns that have ended with mixed results. As a senator, he played a leading role in passing a 1994 crime bill that included a ban on certain semiautomatic weapons. It expired 10 years later.

"I got that done when I was a senator. It passed," Biden said Tuesday. "We should do it again."

Kim reported from Columbus, Ohio. The Washington Post's Tom Hamburger contributed to this report.