Democrats on the House Oversight and Government Reform Committee are calling on committee Chairman Trey Gowdy, R-S.C., to subpoena the White House for documents related to former national security adviser Mike Flynn and his foreign contacts.

"For years, you have warned witnesses that when the Oversight Committee sends a document request, it is not an optional exercise, and they must comply unless they assert a valid privilege," all 18 Democrats on the committee said in a letter Wednesday to Gowdy. "Yet, the White House has been openly defying this Committee's bipartisan request for documents regarding General Flynn for months without any assertion of privilege of any kind."

Flynn's contacts with, and work for, foreign governments have been under scrutiny by numerous congressional committees and special counsel Robert Mueller, who is leading the federal probe into Russian meddling in the 2016 election. Flynn left his post at the White House in February after he lied to Vice President Mike Pence about his contacts with then-Russian Ambassador Sergei Kislyak.

The calls for Gowdy to subpoena the White House, or allow the committee to vote on whether to do so, come after then-Oversight Committee Chairman Jason Chaffetz, R-Utah, and ranking member Elijah Cummings, D-Md., sent a letter in March to then-White House chief of staff Reince Priebus asking for documents related to payments Flynn received from foreign entities.

Chaffetz and Cummings sent the letter to Priebus after they received documents indicating Flynn received more than $67,000 from Russian companies, including $45,000 from RT for his attendance at a gala in Moscow; $11,250 from a Russian charter cargo airline; and $11,250 from a cybersecurity firm based in Russia.

Flynn and his lobbying company also had a $530,000 lobbying contract for work they did on behalf of the Turkish government, which he hadn't previously disclosed.

In April, the White House sent a letter to the committee acknowledging it had the documents the lawmakers requested, but didn't turn anything over. In its letter, the White House did not assert executive privilege.

In addition to pressing Gowdy to subpoena the White House, the Oversight Committee Democrats also urged him to subpoena Flynn's company, Flynn Intel Group, and two others he did business with, ACU Strategic Partners and IP3/IronBridge.

The Democrats cited previous reports indicating Flynn failed to disclose on his application for a security clearance a June 2015 trip to the Middle East and other foreign contacts he had related to "efforts to promote a U.S.-Russian nuclear energy partnership in the Middle East."

Democrats have requested information from the companies and, though they turned over some documents, the companies declined to provide additional information, saying they need a "compulsory" or "formal request" from the entire committee.

"Your decisions on this investigation will have a profound impact on the faith that the American people have in Congress to fulfill our duty under the Constitution to exercise robust oversight of the Executive Branch, especially when grave violations of the public trust are at stake," the Oversight Democrats wrote to Gowdy. "We ask that you compel these entities to produce the requested information, or at the very least, allow the Committee to vote on issuing subpoenas at the next business meeting."