Demonstrators hold up placards representing the number of the people who have died due to gun violence on Capitol Hill in Washington, DC, on June 20, 2019, during an event with gun violence prevention advocates.

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.