Reps. Eliot Engel, Adam Schiff and Elijah Cummings, the chairmen of three House committees, informed the State Department in a fiery letter late Tuesday that Secretary of State Mike Pompeo might now be a "fact witness" in their ongoing impeachment inquiry, after Pompeo accused Dems of trying to "bully" foreign service officers into testifying.

Saying Pompeo "now appears to have an obvious conflict of interest" because he "reportedly participated" in President Trump's fateful July call with Ukraine's president, the Democrats also warned that Pompeo could run afoul of obstruction and whistleblower law -- crimes, the Democrats noted, that could carry multiyear prison terms. A source familiar with the situation told Fox News that Pompeo was indeed in on the July 25 call at the center of the impeachment drive.

Pompeo "should not be making any decisions regarding witness testimony or document production in order to protect himself or the president," the Democrats wrote. "Any effort by the secretary of the department to intimidate or prevent witnesses from testifying or withhold documents from the committees shall constitute evidence of obstruction of the impeachment inquiry."

POMPEO HITS DEMS FOR 'BULLYING' STATE DEPT

The letter continued: "Given the secretary's own potential role, and reports of other State Department officials being involved in or knowledgeable of the events under investigation, the committees may infer that he is trying to cover up illicit activity and misconduct, including by the president. This would be a blatant cover-up and a clear abuse of power.

"The department must immediately halt all efforts to interfere with the testimony of State Department witnesses before Congress."

HOUSE COMMITTEES SUBPOENA POMPEO FOR UKRAINE DOCUMENTS AS PART OF IMPEACHMENT INQUIRY

In letters and tweets on Tuesday, Pompeo informed Democrats that their "requested dates for depositions are not feasible," claiming the lawmakers have not given the employees enough time to prepare and voicing concern that they're trying to prevent State Department counsel from participating in the depositions.

Pompeo said Executive Branch counsel would have to be present.

"Let me be clear: I will not tolerate such tactics, and I will use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals whom I am proud to lead and serve alongside at the Department of State," Pompeo tweeted in response to the Democrats' push for depositions.

Democrats launched their formal impeachment inquiry last month after Trump asked Ukrainian officials over the summer to investigate former Vice President Joe Biden, his son Hunter and the Bidens' business activity in Ukraine. Fox News has confirmed that State Department Inspector General Steve Linick will come to Capitol Hill on Wednesday to meet with House and Senate Committee aides regarding Ukraine.

In a letter last week, the three chairmen notified Pompeo that they had scheduled depositions for five State Department officials between Wednesday and Oct. 10.

Those officials are former Ambassador to Ukraine Marie Yovanovitch; exiting U.S. Special Representative for Ukraine Negotiations Kurt Volker; Deputy Assistant Secretary George Kent; Counselor T. Ulrich Brechbuhl; and U.S. Ambassador to the European Union Gordon Sondland. All of the officials except Kent are named in a complaint by an intelligence community whistleblower who accused Trump of "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."

GIULIANI SAYS POMPEO WAS AWARE OF HIS UKRAINE OUTREACH

"The failure of any of these department employees to appear for their scheduled depositions shall constitute evidence of obstruction of the House's impeachment inquiry," the chairmen wrote.

In a Tuesday tweet, Pompeo posted his letter to Engel and accused the Democrats of trying to “intimidate, bully, & treat improperly” the State employees.

In his letter, Pompeo also accused committee staff of "sending intimidating communications to career department professionals." He asked that requests be sent not to the employees directly, but rather to the department’s legislative affairs office, "as is customary."

The chairmen also subpoenaed Pompeo last Friday for documents related to Ukraine that they said pertained to the impeachment inquiry. "Your failure or refusal to comply with the subpoena shall constitute evidence of obstruction of the House's impeachment inquiry," the chairmen wrote.

On Tuesday, Pompeo wrote in his letter that the department "intends to respond to that subpoena by the noticed return date" of this Friday.

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The Democrats have demanded copies of the transcript of Trump's July 25 call with Ukrainian President Volodymyr Zelensky and a list of State Department officials who might have been involved with the conversation, according to the letter.

The chairmen additionally requested any State Department records about Rudy Giuliani, Trump's personal attorney, and any records relating to U.S. military aid to Ukraine.

Fox News' Alex Pappas contributed to this report.