WASHINGTON — A former top national security adviser to President Donald Trump has arrived on Capitol Hill to testify in the House impeachment inquiry.

Tim Morrison handled Russian and European affairs at the White House's National Security Council before resigning his position a day before Thursday's hearing.

Morrison, a defense hawk and former top Republican staff member on Capitol Hill, was aligned with Trump's former national security adviser John Bolton and raised concerns about the Trump administration's approach to Ukraine.

The impeachment inquiry is focused on Trump's effort to have Ukraine investigate Democrats and a potential 2020 rival, Joe Biden, while the administration was withholding military aid to the Eastern European ally.

It's illegal to seek or receive foreign help in U.S. elections. Trump says he did nothing wrong.

Morrison, who is testifying under subpoena, 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 Biden and meddling in the 2016 election.

Morrison, a political appointee in the Trump White House, brought on board by 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 set to be 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 Morrison says, GOP lawmakers will be hard-pressed to dismiss the former 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 Bill Taylor, the top U.S. diplomat 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 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."