“If subpoenaed, Mr. Morrison plans to appear for his deposition,” his attorney, Barbara Van Gelder, said.

A slew of high-profile witnesses have defied White House, State Department and Pentagon orders not to cooperate with the impeachment probe. In each case, lawmakers have issued a subpoena, which the officials have relied on to justify testifying over the administration’s objections.

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.