"Even if the inquiry was validly authorized, much of the information sought in the subpoena appears to consist of confidential Executive Branch communications that are potentially protected by executive privilege and would require careful review to ensure that no such information is improperly disclosed," wrote Melissa Burnison, assistant Energy secretary for congressional and intergovernmental affairs.

Congress had asked Perry to turn over documents relating to meetings he attended with officials in the Ukraine government, from which the administration had threatened to withhold military aid until they publicly announced an investigation into a natural gas company that had hired the son of former Vice President and current presidential candidate Joe Biden.

Perry has been in the thick of the administration’s Ukraine diplomacy but has maintained that he has known nothing of its search for dirt on the Bidens. The former Texas governor also tried to get a former campaign donor appointed to the board of Ukraine’s state-owned natural gas company.

As the impeachment inquiry further heats up, the Energy Department announced Perry would travel to Belgium, United Arab Emirates and Saudi Arabia next week, and Trump said he would nominate Deputy Secretary of Energy Dan Brouillette to succeed Perry at the agency.

Democrats originally subpoenaed the former Texas governor on Oct. 10 for a host of records related to a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky at the heart of the probe, as well as documents concerning his frequent travel to the country and meetings Perry’s held with Ukrainian officials.

Perry has acknowledged asking Trump to make the July phone call with the Ukrainian president at the heart of the probe, as well as working on the administration's policy with the president’s personal attorney Rudy Giuliani. He’s argued his concerns were centered on general corruption in the country and strongly denied hearing of any effort to seek political ammunition against Biden in exchange for U.S. military support.

“There was no quid pro quo in the sense of what those folks out there would like for it to be: We’re [not] going to give you this money unless you go investigate Joe Biden and his son,” Perry said in a Fox News interview. “I never heard that said anywhere, anytime in any conversation."

Acting White House chief of staff Mick Mulvaney told reporters Thursday, however, that the administration did indeed hold up military funds to Ukraine to obtain a political probe sought by Trump, undercutting prior denials of a quid pro quo.

Calls have been mounting around Capitol Hill among Democrats — and even Republican Rep. Francis Rooney (Fla.) — for Perry to cooperate as part of the inquiry and testify. Democrats on the three committees conducting the probe argue that as one of the “three amigos” responsible for Ukrainian policy in the administration, Perry almost certainly has relevant testimony.

In an interview earlier Friday, Perry said he would defer to Energy Department counsel on whether to comply with the subpoena or offer testimony as part of the impeachment inquiry. He also criticized the “almost crazed idea” of impeachment as well as how Democrats are conducting it.

"If they're going to do this, they need to at least follow the historic rules," Perry said. "And they're not."