Perry's plans after leaving the Energy Department were not immediately known, but the 69-year-old has ruled out another try for the White House after running unsuccessfully in the 2012 and 2016 Republican primaries. “I’m done. Quote me on that,” he said when asked about another presidential campaign last year, adding that he’d “totally failed” at retiring earlier as Texas governor.

But it's an open question how much of his retirement will be spent answering questions about the Ukraine affair, which centers on questions about whether Trump withheld U.S. military aid to pressure the government in Kiev to investigate Democratic rival Joe Biden and his son Hunter.

Perry has drawn scrutiny because he led the U.S. delegation to Ukrainian President Volodymyr Zelensky's inauguration in May, a visit that came as the administration was trying to determine whether the new leader would be amenable to Trump's demands, according to a whistleblower's report that the White House released last week. Perry was a last-minute replacement for Vice President Mike Pence, who is facing mounting questions about his own role in the scandal.

No evidence has emerged that Perry was directly involved with Trump’s attempt to drum up an investigation focused on his political opponent, but Sen. Bob Menendez (D-N.J) earlier this week sent a letter to Perry requesting information about his activities and interactions there.

The White House did not comment on Perry's future, and a DOE spokesperson declined to say whether he would resign next month.

“While the Beltway media has breathlessly reported on rumors of Secretary Perry's departure for months, he is still the Secretary of Energy and a proud member of President Trump’s Cabinet. One day the media will be right. Today is not that day,” DOE spokesperson Shaylyn Hynes said.

Brouillette has been filling in for Perry at Cabinet meetings for the past few months, one source added. Many of Perry’s former DOE staff members — including chief of staff Brian McCormack and special assistant Luke Wallwork — have all left DOE in recent weeks, a source said.

Perry, a frequent traveler to Eastern Europe as pitchman for U.S. energy exports, was also a subject in the subpoena that House Democrats served to Trump’s personal lawyer Rudy Giuliani earlier this week. The subpoena includes a demand for documents and other communications involving Perry and the former New York City mayor connected to Ukraine. A second subpoena expected to be issued this week will seek details of conversations between acting White House chief of staff Mick Mulvaney and Perry, as well as records from other current or former DOE officials.

Perry had been sharply critical of Trump in 2015, calling his then-rival's campaign "a cancer on conservatism." But Trump nevertheless tapped him to run the Energy Department — an agency Perry once pledged to shut down had he been successful in his White House bid. He came to D.C. wary of getting caught up in the sort of scandals that eventually forced out former Environmental Protection Agency Director Scott Pruitt and former Interior Secretary Ryan Zinke, according to multiple people close to him.

Perry eagerly took the lead in Trump's effort to resurrect the struggling coal industry, but his bid to persuade energy regulators to establish financial support for coal power plants was soundly rejected by the bipartisan Federal Energy Regulatory Commission. He shifted his attention to promoting U.S. supplies of coal, oil and natural gas to foreign governments, positioning U.S. energy supplies as a counterbalance to Russian and OPEC exports. Still, his earnestness often drew mockery, including his references to American natural gas as "molecules of U.S. freedom."

But he proved to be a successful promoter of liquefied natural gas exports, traveling regularly to Poland, Ukraine, Lithuania and other Eastern European countries to pitch exports.

"We're going to bring our A-game," he said after a 10-day trip to Eastern Europe in 2018. "We're going to try to win every contract that we can, knowing that we can't win every contract and we can't supply every contract. But if we're in the game in a very substantive way, we will help drive the competition, which will drive down the cost of gas."

He also often touted the fuel's role in cutting U.S. greenhouse gas emissions, even though he was criticized by Democrats and environmentalists for rejecting the scientific data showing carbon dioxide was the main factor in driving climate change. As recently as August, he ridiculed Democrats for living in a "fantasy world" in their calls for aggressive action to fight climate change.

Zack Colman and Darius Dixon contributed to this report.