WASHINGTON – Secretary of State Mike Pompeo has an "obvious conflict of interest" in the Ukraine scandal and will likely be a key witness in the congressional impeachment inquiry, House Democrats said Tuesday.

Pompeo may be involved in a "blatant cover-up and a clear abuse of power," a trio of Democratic chairman wrote in a letter Tuesday night.

Given Pompeo's potential role, he should "not be making any decisions regarding witness testimony or document production in order to protect himself or the president," the lawmakers said.

The extraordinary warning – sent to the State Department's deputy secretary of state, John Sullivan – came after a bitter exchange between Pompeo and House Democrats over the scheduled depositions of five State Department officials involved in communications between Rudy Giuliani, Trump's personal lawyer, and Ukrainian government officials.

Giuliani and Trump have pressured Ukraine's new government, led by President Volodymyr Zelensky, to investigate former Vice President Joe Biden and his son, Hunter. Giuliani has said he contacted Zelensky's top adviser at the direction of the State Department and has kept officials at that agency abreast of his conversations.

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Pompeo has come under increasing scrutiny for his role in Trump's pressure campaign. The secretary of state was reportedly listening in to the July 25 phone call between Trump and Zelensky, in which Trump personally lobbied the Ukrainian leader to probe the Bidens. Ukrainian officials have said there is no evidence that Joe Biden or his son did anything wrong.

Pompeo has so far dodged questions about his involvement in the matter, saying only that State Department officials acted "appropriately" in their contacts with Ukrainian officials.

On Tuesday, Pompeo pushed to delay testimony by five State Department officials who House Democrats have asked to depose. He accused Democrats of trying to "intimidate" and "bully" career professionals in their quest to impeach Trump.

"I’m concerned with aspects of the committee’s request that can be understood only as an attempt to intimidate, bully and treat improperly the distinguished professionals of the Department of State," Pompeo wrote in a letter to House Foreign Affairs Committee Chairman Eliot Engel earlier Tuesday.

He said the committee's request to depose five State Department employees this week and next week is "not feasible" – throwing a wrench into Democrats' plans to move quickly with their inquiry.

Engel and other Democrats responded by accusing Pompeo of "stonewalling" the impeachment inquiry. They escalated those charges Tuesday night amid new reports that the State Department's inspector general wanted to brief lawmakers on an "urgent" concern related to Ukraine.

More:Democrats accuse Pompeo of 'stonewalling,' after he pushes back on impeachment requests

Democrats argued that Pompeo seemed to be interfering with testimony that could prove pivotal to the impeachment inquiry.

"This would be a blatant cover-up and a clear abuse of power," they wrote.

The State Department's press office did not immediately respond to a request for comment.