Morrison, however, would be the first currently serving White House official to testify. He’s also the first official believed to be on a July 25 phone call between President Donald Trump and Ukrainian President Volodymyr Zelensky during which Trump pressed his counterpart to investigate former vice president Joe Biden.

Morrison was also a crucial figure identified Tuesday by Trump’s ambassador to Ukraine, William Taylor, as a witness to Trump’s effort to withhold military aid from Ukraine in order to bend Zelensky to his will.

Taylor testified that on Sept. 1, Morrison told him about a conversation he witnessed between Trump's E.U. ambassador Gordon Sondland and a senior Ukrainian government official. In that conversation, Taylor said Morrison told him, Sondland informed the official that hundreds of millions of dollars in U.S. military aid to Ukraine were dependent on the opening of political investigations.

Taylor also said Morrison spoke to him on Sept. 7 about another conversation with Sondland. In that conversation, per Taylor's recounting, Sondland revealed he had spoken directly with Trump about Ukraine and that Trump insisted that Zelensky should publicly declare the political investigations himself, rather than leaving it to subordinates. According to Taylor, Morrison said he then relayed details of the call to then-national security adviser John Bolton as well as NSC lawyers.

Van Gelder declined to comment on a CNN report that indicated Morrison would corroborate the key details of Taylor's testimony. "We will not be commenting on what he will say to the Committees," she said.

Sondland already testified to lawmakers last week but some Democrats have demanded he return for further questioning in light of Taylor's testimony.

