Though the letter did not mention the Johnson investigation, it included a classified addendum that the two sources say identified the probe as one of the sources of their concern.

“We are gravely concerned, in particular, that Congress appears to be the target of a concerted foreign interference campaign, which seeks to launder and amplify disinformation in order to influence congressional activity, public debate, and the presidential election in November,” Pelosi (D-Calif.) and Schumer (D-N.Y.) wrote in their letter, which was also signed by the chair of the House Intelligence Committee, Rep. Adam Schiff (D-Calif.), and the vice chair of the Senate Intelligence Committee, Sen. Mark Warner (D-Va.).

All four Democratic signatories are members of the Gang of Eight, a group of eight lawmakers who are briefed on classified intelligence by the executive branch.

A spokesman for Johnson, who chairs the Senate Homeland Scrutiny and Governmental Affairs Committee, did not immediately respond to a request for comment.

The letter’s authors did not publicly offer additional details about the nature of the counterintelligence threat they described. But lawmakers of both parties have warned for months that foreign powers like Russia are intent on interfering in the 2020 U.S. election process — including, Democrats say, through Johnson’s investigation, which targets Biden’s son Hunter.

The probe centers on claims that a Democratic public-relations firm sought to leverage Hunter Biden’s role on the board of a Ukrainian energy company, Burisma, to influence the State Department under the Obama administration. Johnson has asked several former State Department officials to testify, and he is eyeing subpoenas as soon as this week if they do not agree to appear for depositions voluntarily.

Johnson renewed his demand for transcribed interviews and documents from the former officials days after a Ukrainian lawmaker — Andriy Derkach, who has met with Trump’s personal lawyer Rudy Giuliani to discuss investigating the Biden family — used a news conference to accuse the Bidens and Amos Hochstein, a former special envoy for international energy affairs at the State Department, of an elaborate conspiracy to steal hundreds of millions of dollars from Ukraine.

The last time senators were briefed on election security and foreign influence operations, Democratic lawmakers confronted Johnson behind closed doors about his investigation, arguing that it threatens the integrity of the 2020 election and relies on Russian disinformation to tar a political opponent.

They cited in particular Johnson’s initial effort to subpoena Andrii Telizhenko, who has pushed unsubstantiated claims about coordination between the Ukrainian government and the Democratic National Committee in 2016. Johnson dropped plans to subpoena Telizhenko after the FBI’s foreign influence task force briefed senators about him, focusing on concerns over his credibility.

Last week, Sen. Gary Peters of Michigan, the top Democrat on the Homeland Security Committee, renewed his request for defensive briefings from the FBI as Johnson’s investigation intensifies.

Some Senate Republicans, too, have previously signaled unease with Johnson’s investigation. In December, Sen. Richard Burr (R-N.C.), who was chair of the Senate Intelligence Committee at the time, privately told Johnson that his inquiry could aid Russia, according to two congressional sources familiar with the meeting. And Sen. Lindsey Graham (R-S.C.) warned in February that any derogatory information coming out of Ukraine about any American should be vetted by intelligence agencies because “Russia is playing us all like a fiddle.”

The Democratic leaders’ letter was dated July 13 and addressed to FBI Director Christopher Wray, but they released it publicly Monday.

“Given the seriousness and specificity of these threats, as members of congressional leadership and the congressional intelligence committees we believe it is imperative that the FBI provide a classified defensive briefing to all Members of Congress and that the briefing draw on all-source intelligence information and analysis, consistent with due regard for the protection of sensitive intelligence sources and methods,” they continued.

The FBI has acknowledged receiving the letter, but the bureau declined to comment further. In their letter, the Democratic leaders demanded the briefing “prior to the August recess at the earliest possible opportunity,” and they want a schedule in place by the end of the day Monday.