Tim Morrison, a top National Security Council (NSC) official, is the first White House official slated to testify as part of Democrats' impeachment inquiry that is examining President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's contacts with Ukraine.

An attorney for Morrison, who took over Fiona Hill’s role as the senior director for European and Russian affairs on the NSC, says her client intends to testify if he is subpoenaed, even if the White House seeks to block him from testifying.

“If subpoenaed, Mr. Morrison plans to appear for his deposition,” attorney Barbara Van Gelder said in a statement, while declining to preview what he will say in his upcoming testimony.

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Morrison is seen as a key witness by Democrats.

He is believed to be involved on the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky, during which Trump pressed the foreign leader to open an investigation into one of his top 2020 political rivals, former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

Morrison is also seen as an official who can corroborate the testimony of William Taylor, the top U.S. diplomat to Ukraine who offered what Democrats described as the "most powerful" testimony earlier this week.

Taylor in his testimony said he understood that Trump was withholding nearly $400 million in financial aid to Ukraine in order to secure a commitment from Zelensky that he would investigate interference in the 2016 election as well as Burisma, a Ukrainian energy company that employed Biden’s son Hunter Biden.

In his opening statement, which was obtained by The Hill and other media outlets, Taylor detailed how Morrison relayed to him several conversations related to the Trump administration demanding a quid pro quo in exchange for the Ukraine aid.

“During this same phone call I had with Mr. Morrison, he went on to describe a conversation Ambassador Sondland had with Mr. Yermak at [a meeting in] Warsaw,” Taylor said in his detailed 15-page opening remarks. “Ambassador Sondland told Mr. Yermak that the security assistance money would not come until President Zelenskyy committed to pursue the Burisma investigation.”

Taylor also said Morrison later described to him a “sinking feeling” in early September, after Trump told U.S. Ambassador to the European Union Gordon Sondland there was no “quid pro quo,” all while continuing to insist that Zelensky should publicly announce that he is “opening investigations of Biden and the 2016 election interference, and that President Zelensky should want to do this himself.”

Morrison's testimony, which is slated for Thursday, comes after a series of other career officials from the State Department and other agencies defied White House orders seeking to block them from testifying. Morrison's testimony will also fall on a jam-packed week with four other witnesses who are also expected to testify behind closed doors.

House Democrats have issued subpoenas to compel their testimony, giving witnesses cover to come testify.