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(CNN) —  

Tim Morrison, a top Russia and Europe adviser on President Donald Trump’s National Security Council, is expected to testify before House impeachment investigators next week and corroborate key elements of a top US diplomat’s account that Trump was pressing for Ukraine to publicly announce investigations into the Bidens before he would greenlight US security assistance, according to sources.

Bill Taylor, the top US diplomat in Ukraine, said in extraordinary testimony on Tuesday that Trump pushed for Ukraine to publicly announce investigations, including one into former Vice President Joe Biden and his son, using as leverage the military aid the country sought to fight back against Russian aggression.

Morrison’s testimony is expected to be significant because he is a current White House official whose name was cited 15 times in Taylor’s opening statement, which Democrats view as damning for Trump.

Morrison also listened to the July 25 call between Trump and the Ukrainian leader, CNN reported earlier this month. His testimony would be one of the first from someone who heard the call directly. A transcript of the call was released by the White House but was not a full verbatim.

But two sources also tell CNN that Morrison will contend that he didn’t see anything wrong with what the Trump administration did, while one of the sources said there will be “nuance” over what Morrison intends to say.

Morrison, whose notes are expected to form the basis of his testimony, has told associates he wants to cooperate with the probe – even though the White House has railed against an inquiry it has contended is “illegitimate.”

Nevertheless, Republicans in the White House and in Congress are worried that Morrison could provide testimony that will fuel the Democrats’ impeachment push, according to sources familiar with the matter. The GOP is hoping that Morrison’s testimony will lack firsthand knowledge of the President’s actions, but it’s unclear yet what the White House official will say.

Morrison’s attorney told CNN that it was premature to discuss her client’s testimony since he hasn’t decided what he’s going to say.

“We haven’t decided what he’s going to say yet,” Barbara Van Gelder, an attorney for Morrison, said Thursday. “He’s going to answer questions. That’s where we’re at right now. We haven’t made final decisions on anything other than he’ll appear with the subpoena.”

Asked if that meant he would definitely appear next week, Van Gelder said: “Currently, yes.”

Morrison informed Taylor about the now-famous July 25 call between Trump and Ukrainian President Volodymyr Zelensky, which became public after a government whistleblower provided a complaint now at the center of the impeachment inquiry. Morrison told Taylor that the call “could have been better,” according to Taylor’s testimony.

In August, Morrison told Taylor that Trump “doesn’t want to provide any assistance at all” to Ukraine, even though Congress had already appropriated it, according to Taylor’s testimony. The diplomat said he had found that claim “extremely troubling” and prepared to resign after his call with Morrison.

Taylor also referenced subsequent conversations he had with Morrison amid the push to get the White House to release military aid to Ukraine that had been approved by Congress. Taylor testified that he was “alarmed” to learn from Morrison that the Trump administration “conditioned” not only a Trump-Zelensky White House meeting but also the military assistance on the investigations.

Taylor said that Morrison told him about a conversation between Gordon Sondland, the American ambassador to the European Union, and Andriy Yermak, a Zelensky aide. Sondland told Yermak in September that the aid “would not come” until Zelensky “committed to pursue the Burisma investigation,” according to Taylor’s testimony. Burisma is the Ukrainian natural gas company on whose board Hunter Biden sat while his father was vice president.

Later, Morrison told Taylor he had a “sinking feeling” about a conversation on September 7 between Trump and Sondland, according to Taylor. Earlier that day, Trump had told Sondland he was not asking for a “quid pro quo” from Ukraine. But Taylor’s testimony said that Trump wanted Zelensky to “go to a microphone” and say that he is opening investigations into Biden and 2016 election interference – and “that President Zelensky should want to do this himself.”

The Trump administration withheld nearly $400 million in military aid before the July phone call between Trump and Zelensky. The administration ultimately released the aid in September.

Democrats have charged that Trump abused his power, an impeachable offense, in using his public office for personal gain, asking a foreign country to interfere in the next presidential election. Trump and Republicans rebut that the impeachment investigation is a witch hunt, arguing that there was no “quid pro quo” between Trump and Zelensky since Ukraine eventually received the aid.

CLARIFICATION: This article has been updated to reflect that Morrison’s testimony would be one of the first from someone on the call between the president and the Ukrainian leader. Another person who reportedly listened on the call, Charles Kupperman, could testify next week too.