Former top national security adviser to President Donald Trump, Tim Morrison, arrives this morning for a closed door meeting to testify as part of the House impeachment inquiry into President Donald Trump on Capitol Hill in Washington. (AP Photo/Andrew Harnik)

WASHINGTON (AP) — President Donald Trump’s former top adviser for Russian and European affairs arrived on Capitol Hill today to testify to House impeachment investigators , a day after leaving his job at the White House.

Tim Morrison, the first White House political appointee to testify, didn’t respond to reporters’ questions about his testimony, which takes place behind closed doors, but his information might be central to a push to remove the president from office.

Morrison, who served on the National Security Council, stepped down from that post Wednesday, and a senior administration official said he “decided to pursue other opportunities.” The official, who was not authorized to discuss Morrison’s job and spoke only on the condition of anonymity, said Morrison has been considering leaving the administration for “some time.”

He has been in the spotlight since August when a government whistleblower said multiple U.S. officials had said Trump was “using the power of his office to solicit interference from a foreign country in the 2020 U.S. election.”

Morrison will be asked to explain that “sinking feeling” he got when Trump demanded that Ukraine’s president investigate former Vice President Joe Biden and meddling in the 2016 election.

Morrison was brought on board by then-national security adviser John Bolton to address arms control matters and later shifted into his current role as a top Russia and Europe adviser. It was there that he stepped into the thick of an in-house squabble about the activities of Trump’s personal attorney Rudy Giuliani, who had been conversing with Ukrainian leaders outside of traditional U.S. diplomatic circles.

Known as a “hawk” in national security circles, Morrison is the first political appointee from the White House to testify before impeachment investigators. The probe has been denounced by the Republican president, who has directed his staff not to testify.

Regardless of what he says, GOP lawmakers will be hard-pressed to dismiss Morrison, formerly a longtime Republican staffer at the House Armed Services Committee. He’s been bouncing around Washington in Republican positions for two decades, having worked for Rep. Mark Kennedy, R-Minn., Sen. Jon Kyl, R-Ariz., and as a GOP senior staffer on the House Armed Services Committee, including nearly four years when it was chaired by Rep. Mac Thornberry, R-Texas.

Morrison’s name appeared more than a dozen times in earlier testimony by William Taylor, the acting U.S. ambassador in Ukraine, who told impeachment investigators that Trump was withholding military aid unless the new Ukrainian president, Volodymyr Zelenskiy, went public with a promise to investigate Trump’s political rival Joe Biden and his son Hunter. Taylor’s testimony contradicts Trump’s repeated denials that there was any quid pro quo.

Taylor said Morrison recounted a conversation that Gordon Sondland, America’s ambassador to the European Union, had with a top aide to Zelenskiy named Andriy Yermak. Taylor said Morrison told him security assistance would not materialize until Zelenskiy committed to investigate Burisma, a Ukrainian gas company that once employed Biden’s son. A White House meeting for Zelenskiy also was in play.

“I was alarmed by what Mr. Morrison told me about the Sondland-Yermak conversation,” Taylor testified. “This was the first time I had heard that the security assistance — not just the White House meeting — was conditioned on the investigations.”

Taylor testified that Morrison told him he had a “sinking feeling” after learning about a Sept. 7 conversation Sondland had with Trump.

“According to Mr. Morrison, President Trump told Ambassador Sondland that he was not asking for a quid pro quo,” Taylor testified. “But President Trump did insist that President Zelenskiy go to a microphone and say he is opening investigations of Biden and 2016 election interference, and that President Zelenskiy should want to do this himself. Mr. Morrison said that he told Ambassador Bolton and the NSC lawyers of this phone call between President Trump and Ambassador Sondland.”

Morrison told people after Bolton was forced out of his job that the national security adviser had tried to stop Giuliani’s diplomatic dealings with Ukraine and that Morrison agreed, according to a U.S. official, who was not authorized to discuss Morrison’s role in the impeachment inquiry and spoke only on condition of anonymity. The official said Morrison told people that with the appointment of Robert O’Brien as Bolton’s successor, his own future work at the NSC was in a “holding pattern.”

Bolton had brought Morrison into the NSC in July 2018 as senior director for weapons of mass destruction and biodefence. He’s known as an arms control expert or an arms treaty saboteur, depending on who you ask.

Morrison, who earned a bachelor’s degree from the University of Minnesota and a law degree from George Washington University, keeps nuclear strategist Herman Kahn’s seminal volume on thermonuclear warfare on a table in his office.