Morrison, Williams and Vindman were part of the small cadre of White House staffers who listened to Trump's July 25 phone call with the newly elected Ukrainian president, Volodymyr Zelenskiy.

Four witnesses are scheduled to testify in two separate hearings, beginning at 9 a.m. ET. The first hearing will feature testimony from Lt. Colonel Alex Vindman, a Ukraine expert at the National Security Council, and Jennifer Williams, a European security expert working for Vice President Mike Pence . Following Vindman and Williams, the former top Russia and Europe expert at the NSC, Tim Morrison, will testify, along with Trump's former special envoy to Ukraine, Kurt Volker.

After an opening week of testimony from diplomats who described how U.S. policy toward Ukraine came to be dominated by President Donald Trump's political goals, the House Intelligence Committee on Tuesday shifts to Trump's July 25 phone call with Ukraine's president during which Trump asked him to investigate Joe Biden.

The call prompted an anonymous whistleblower complaint from an intelligence analyst who expressed concerns that Trump's request for Ukraine to conduct specific investigations into American citizens posed a threat to U.S. national security.

Vindman has already testified behind closed doors that he thought the president's request was "improper," and that he reported his concerns to the NSC's top lawyer. Vindman also testified that the White House transcript of the call left out key lines. They included an unfounded claim Trump made to Zelenskiy about former Vice President Biden, and a specific reference Zelenskiy made to Burisma, a Ukrainian energy company whose board included Biden's son Hunter.

Vindman received a Purple Heart after he was wounded in action during the Iraq War. Nonetheless, Republicans have signaled that they plan to attack Vindman's personal motives for testifying and question his patriotism.

On the eve of Vindman's public testimony, Republican Sen. Ron Johnson accused Vindman, without evidence, of being part of an anti-Trump "deep state" conspiracy. The Wisconsin senator also wrote in a public letter he released Monday night that it was "entirely possible" that Vindman "fit the profile" of someone "participating in the ongoing effort to sabotage [Trump's] policies and, if possible, remove him from office."

Testifying alongside Vindman on Tuesday will be Williams, who is expected to say that she, like Vindman, thought Trump's demand for investigations from Ukraine during the July call was "unusual and inappropriate."

Williams is likely to face less aggressive questioning from Republicans than Vindman will, since she was less involved in Ukraine policy. Republicans could even use her testimony to argue that Pence was largely removed from the Ukraine pressure campaign, thereby suggesting that the effort was confined to a few individuals, and not widespread administration policy.

Tuesday afternoon will feature testimony from Morrison and Volker, two of the only people testifying this week whose names were on a list of witnesses requested by Republican members of the committee.

A longtime Republican congressional staffer before he joined the White House, Morrison resigned from the NSC last month, one day before he testified behind closed in the impeachment probe. Morrison is one of the only witnesses interviewed so far who says he did not hear Trump say anything "improper" during the July 25 call.

While Republicans are likely to use Morrison's characterization of the call as a springboard to defend Trump, he is also expected to testify that there were clearly defined conditions placed on the U.S. aid to Ukraine: namely, funds would remain on hold until Ukraine announced investigations into Biden and the Democratic National Committee.

The fourth witness, Volker, resigned from the Trump administration shortly before he gave testimony to House investigators on Oct. 4.

Volker and Sondland, together with Energy Secretary Rick Perry, were dubbed "The Three Amigos," and specifically tasked by the president to work with his personal lawyer, Rudy Giuliani, to persuade Ukraine's government to announce it would investigate possible wrongdoing by the Bidens and the Democratic National Committee.