Democrats are prioritizing action on gun control as they return Monday from Congress' summer vacation.
House Speaker Nancy Pelosi, D-Calif., and Senate Minority Leader Sen. Chuck Schumer, D-N.Y., sent a letter to President Donald Trump on Sunday demanding that the president express his support for universal background checks, a move that would effectively give permission to Senate Republicans to take up the matter.
Lawmakers in the House passed bipartisan legislation earlier in the year that would strengthen federal background check laws, but Senate Majority Leader Mitch McConnell has said he will not let the Senate vote on the measure without a clear indication of Trump's support.
"We implore you to seize this moment when your leadership and influence over Republicans in Congress on the issue of guns is so critical," the two Democrats wrote in the letter.
"Please do not squander it by acceding to NRA-backed proposals or other weak ideas that will do nothing to stop the continuing, horrific spread of gun violence and may, in some cases, actually make our communities less safe," the letter said.
The White House did not respond to a request for comment. McConnell's office referred CNBC to comments the Kentucky Republican made on the syndicated radio program "The Hugh Hewitt Show" last week.
"If the president took a position on a bill so that we knew we would actually be making a law and not just having serial votes, I'd be happy to put it on the floor," McConnell told the conservative radio host Sept. 3. "And the administration is in the process of studying what they are prepared to support, if anything. And I expect to get an answer to that next week."
The Pelosi-Schumer letter, and a news conference scheduled for later Monday on the matter, are indications that gun control will be a top legislative priority for Democrats in the busy remaining days of the congressional session.
A number of deadly mass shootings over the summer, including back-to-back massacres in Texas and Ohio in August, energized activists and lawmakers who have long sought to impose new regulations on gun ownership.
The two Democrats cited another shooting, an Aug. 31 mass shooting in Odessa, Texas, that left seven dead, in their letter to the president. Pelosi and Schumer wrote that the shooter in that case was able to obtain the weapon he used through a loophole in existing law, related to background check requirements for private gun sellers, that the background check bill would close.
"While closing this loophole will not stop every shooting, it will undoubtedly save many lives by helping to keep guns out of the hands of dangerous individuals," the Democrats wrote.
Surveys show that more than 4 in 5 Americans support background checks for every gun buyer. But even in the wake of the summer tragedies, less than 25% of Americans expect action from Congress, a new poll from USA Today and Suffolk University found.
Trump has signaled that he is open to new background check laws, but has not endorsed any specific legislation.
"I have an appetite for background checks," Trump told reporters last month. "We're going to be doing background checks. We're working with Democrats. We're working with Republicans. We already have very strong background checks, but we're going to be filling in some of the loopholes."
But the president has also held talks with leaders from the National Rifle Association, a pro-gun lobby that opposes new federal background check rules.
In a tweet posted Aug. 9, Trump wrote that "Serious discussions are taking place between House and Senate leadership on meaningful Background Checks. I have also been speaking to the NRA, and others, so that their very strong views can be fully represented and respected."
Some Republicans in the Senate, including members of the party's leadership, have pushed the president to stake out a clearer position.
In an interview on NBC's "Meet the Press" on Sunday, Sen. Roy Blunt, R-Mo., called on the president to take action,.
"The president needs to step up here and set some guidelines for what he would do," Blunt, chairman of the Senate Republican Policy Committee, said.