Senate Democratic leaders will send a letter to Secretary of State Mike Pompeo and Treasury Secretary Steven Mnuchin on Monday afternoon demanding they sanction Russia — and potentially Russian President Vladimir Putin himself — for attempting to influence the 2020 presidential election.

Why it matters: The letter follows reports that a senior intelligence official briefed Congress that Russia is again interfering in the November election to help Trump. White House national security adviser Robert O'Brien repeatedly rejected that assessment on Sunday, and CNN later reported that the briefer may have overstated the intelligence community's evidence about Russia's goals.

Meanwhile, Sen. Bernie Sanders told reporters Friday that U.S. intelligence officials briefed him "about a month ago" on Russia's attempts to boost his campaign in the Democratic primary.

Trump seized on this in a tweet over the weekend, suggesting Democrats should call on former special counsel Robert Mueller to investigate Sanders' triumph in Nevada on Saturday.

over the weekend, suggesting Democrats should call on former special counsel Robert Mueller to investigate Sanders' triumph in Nevada on Saturday. "Are any Democrat operatives, the DNC, or Crooked Hillary Clinton, blaming Russia, Russia, Russia for the Bernie Sanders win in Nevada. If so I suggest calling Bob Mueller & the 13 Angry Democrats to do a new Mueller Report, Democrat Edition. Bob will get to the bottom of it!" Trump tweeted.

Details: Senate Minority Leader Chuck Schumer, Senate Foreign Relations Ranking Member Bob Menendez and Sen. Sherrod Brown urge Pompeo and Mnuchin in the letter "to identify and target for sanctions all those determined to be responsible for ongoing elections interference, including President Putin."

"Doing anything less would be an abdication of your responsibility to protect and defend the US from this serious threat to our national security, and to the integrity of our electoral process," the letter reads.

Full letter: