Fiona Hill, a former Russian Federation expert on the National Security Council, is scheduled to speak on Thursday morning.
Ambassador Kurt Volker, who is a former envoy to the Ukraine.
The witnesses will then be sworn in and get to deliver their opening statements.
"Anticipating a very busy 2020, the President is taking advantage of a free weekend here in Washington, D.C., to begin portions of his routine annual physical exam at Walter Reed", Grisham said in a statement on Saturday.
Both listened in on Trump's key July 25 phone call with Zelensky, in which the U.S. leader pressured his counterpart to launch investigations into Biden and a wholly unsupported theory that Kiev aided Democrats in the 2016 race.
At 2:30 p.m. ET, the committee is scheduled to start its second hearing with Morrison and Volker - though that is contingent on the first hearing wrapping up on time. Three State Department officials have already testified about their concerns as Trump pushed Ukraine to investigate Democrats and withheld military aid from the country.
"He disagreed with the president", tweeted Trump campaign communications director Tim Murtaugh.
Vindman and Volker have clashed, for example, over a July 10 White House meeting.
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I always felt positive with the trade and the front office. "I never went in with a negative mindset. That is the bone at the tip of the thumb, above the joint, and it is on Lowry's non-shooting hand.
Trump said on Twitter that he would "strongly consider" House of Representatives Speaker Nancy Pelosi's suggestion that he testify in the House impeachment inquiry into whether he pressured Ukraine to investigate domestic political opponent Joe Biden.
"There was no ambiguity", said Vindman, an Army officer detailed to the White House who showed up at the deposition in his military uniform.
Pence's role throughout the impeachment inquiry has been unclear.
Trump described the proceedings as a "kangaroo court" led by "Little Shifty Schiff", criticizing the Democrats for not allowing White House lawyers to cross-examine the witnesses.
Vindman declined to tell lawmakers who in the intelligence community he may have spoken to after he listened in to a July call between Trump and Zelenskiy.
Dr. John Sotos, a cardiologist who studies the medical history of U.S. presidents, said he was reassured by the memo on Trump's health and that "the most serious concerns are largely eliminated".
Democrats signalled they would plow ahead with the inquiry, scheduling nine witnesses this week.
The President's motorcade drove to the medical center unannounced, with reporters under direction not to report his movement until they arrived Saturday at Walter Reed.
Morrison was among those who heard the Trump-Zelenskiy call firsthand when it happened, and although he testified that he was concerned about what might have happened if it became public, he saw nothing illegal.