WASHINGTON (AP) — A former top White House official who raised concerns about President Donald Trump’s efforts to push Ukraine to investigate his political rivals confirmed some aspects of earlier reports but contradicted others during testimony Thursday before House impeachment investigators .

Tim Morrison, who stepped down from the National Security Council the day before his appearance, is the first White House political appointee to testify and could be central to the effort to remove Trump from office.

Morrison largely confirmed much of what a top diplomat, William Taylor, said in earlier testimony, as the two had multiple phone conversations raising concerns about the Trump administration’s approach toward Ukraine, according to a person familiar with the closed-door testimony. The person, who was not authorized to discuss the proceedings, spoke on condition of anonymity.

Morrison also said he warned Taylor of Trump’s plan to withhold security aid and a White House visit from Ukraine unless the country investigated the Bidens and a 2016 election conspiracy theory, according to a source who spoke to the Washington Post.

Republican lawmakers portrayed Morrison’s opening remarks as shifting the debate favorably toward Trump.

They said Morrison, in his opening statement, contradicted another key witness, Lt. Col. Alexander Vindman, the Army officer who handled Ukraine issues at the National Security Council. Vindman testified Tuesday that he twice sounded the alarm over the Trump administration actions.

“It’s a very compelling witness today that is giving testimony that contradicts some of the testimony we heard from Mr. Vindman,” said Rep. Mark Meadows, R-N.C. Morrison’s opening remarks were not publicly released.

“Mr. Morrison’s testimony is very damaging to the Democrat narrative,” Meadows said. “They’ve all of a sudden gotten quiet today because this particular witness is very credible and has given evidence that suggests some of the other witnesses have been less than candid.”

Another Republican, Rep. Chip Roy of Texas, said, “When you all see what he had to say, it will be interesting.”

Morrison, a defense hawk well known in GOP policy circles, was the National Security Council’s top adviser for Russian and European affairs until he stepped down Wednesday. A senior administration official said he had “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.”

Morrison was expected to be asked to explain the “sinking feeling” that he reportedly got when Trump demanded that Ukraine’s president investigate former Vice President Joe Biden and interfere in the 2016 election. The national security hawk, brought on board by then-national security adviser John Bolton , has been featured prominently in previous testimony from diplomat Taylor .

It was Morrison who first alerted Taylor to concerns over Trump’s phone call with Ukrainian President Volodymyr Zelenskiy.

In fact, Morrison’s name appeared more than a dozen times in testimony by Taylor, who told impeachment investigators that Trump was withholding military aid unless Zelenskiy went public with a promise to investigate Trump’s political rival Biden and Biden’s son Hunter. Taylor’s testimony contradicted Trump’s repeated denials that there was any quid pro quo.

Morrison and Taylor spoke at least five times in the weeks following the July phone call as the defense expert and the diplomat discussed the Trump administration’s actions toward Ukraine, according to Taylor’s testimony.

As the security funds for Ukraine were being withheld, Morrison told the diplomat, “President doesn’t want to provide any assistance at all.”

Their concerns deepened when Morrison relayed on Sept. 7 the conversation he had with Ambassador Gordon Sondland a day earlier that gave him that “sinking feeling.” In it, Sondland explained that Trump said he was not asking for a quid pro quo but insisted that Zelenskiy “go to a microphone and say he is opening investigations of Biden and 2016 election interference,” Taylor testified last week.

Morrison told Bolton and the NSC lawyers of this call between Trump and Sondland, according to Taylor’s testimony.

The spotlight has been on Morrison 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 was brought on board to address arms control matters and later shifted into a role as a top Russia and Europe adviser. It was then 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.

The impeachment 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 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 the 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 brought Morrison into the NSC in July 2018 as senior director for weapons of mass destruction and biodefence.