Lt. Col. Alexander Vindman, an Army officer at the National Security Council, and Jennifer Williams, his counterpart at Vice President Mike Pence's office, say they had concerns as Trump spoke on July 25 with the newly elected Ukraine president about political investigations into Joe Biden.
After they appear Tuesday morning, the House will hear in the afternoon from former NSC official Timothy Morrison and Kurt Volker, the former Ukraine special envoy.
In all, nine current and former U.S. officials are testifying as the House's impeachment inquiry accelerates. Democrats say Trump's pressure on Ukraine to investigate his Democratic rivals as he withheld U.S. military aid Ukraine needed to resist Russian aggression may be grounds for removing the 45th president.
Trump says he did no such thing and the Democrats just want him gone.
Vindman and the other witnesses have testified in earlier, closed-door sessions. Their depositions have been publicly released, and they'll face direction questions from lawmakers on Tuesday.
"I did not think it was proper to demand that a foreign government investigate a U.S. citizen," said Vindman, a decorated Iraq War veteran. He said there was "no doubt" what Trump wanted from Ukrainian President Volodymyr Zelenskiy.
It wasn't the first time Vindman, a 20-year military officer, was alarmed over the administration's push to have Ukraine investigate Democrats, he testified.
Earlier, during an unsettling July 10 meeting at the White House, Ambassador Gordon Sondland told visiting Ukraine officials that they would need to "deliver" before next steps, which was a meeting Zelenskiy wanted with Trump, the officer testified.
"He was talking about the 2016 elections and an investigation into the Bidens and Burisma," Vindman testified, referring to the gas company in Ukraine where Hunter Biden served on the board.
"The Ukrainians would have to deliver an investigation into the Bidens," he said. "There was no ambiguity."
On both occasions, Vindman said, he took his concerns about the shifting Ukraine policy to the lead counsel at the NSC, John Eisenberg.
Williams, a longtime State Department official who is detailed to Pence's national security team, said she too had concerns during the phone call, which the aides monitored as is standard practice.
When the White House produced a rough transcript later that day, she put it in the vice president's briefing materials. "I just don't know if he read it," Williams testified in a closed-door House interview.
Pence's role throughout the impeachment inquiry has been unclear, and the vice president's aide is sure to be questioned by lawmakers looking for answers.
Trump has already attacked Williams, associating her with "Never Trumpers," even though there is no indication the career State Department official has shown any partisanship.
The president wants to see a robust defense by his GOP allies on Capitol Hill, but so far so far Republicans have offered a changing strategy as the fast-moving probe spills into public view. They're expected to mount a more aggressive attack on the witnesses as they try to protect Trump.
In particular, Republicans are expected to try to undercut Vindman, suggesting he reported his concerns outside his chain of command, which would have been Morrison, not the NSC lawyer.
Under earlier questioning, Republicans wanted Vindman to disclose who else he may have spoken to about his concerns, as the GOP inch closer to publicly naming the still anonymous whistleblower whose report sparked the inquiry.
GOP Sen. Ron Johnson, who was deeply involved in other White House meetings about Ukraine, offered a sneak preview of this strategy late Monday when he compared Vindman, a Purple Heart veteran, to the "bureaucrats" who "never accepted Trump as legitimate."
"They react by leaking to the press and participating in the ongoing effort to sabotage his policies and, if possible, remove him from office." It is entirely possible that Vindman fits this profile, said Johnson, R-Wis.
The White House has instructed officials not to appear, and most have received congressional subpoenas to compel their testimony.
The witnesses are testifying under penalty of perjury, and Sondland already has had to amend his earlier account amid contradicting testimony from other current and former U.S. officials.
Sondland, the wealthy donor whose routine boasting about his proximity to Trump has brought the investigation to the president's doorstep, is set to testify Wednesday. Others have testified that he was part of a shadow diplomatic effort with the president's personal lawyer, Rudy Giuliani, outside of official channels that raised alarms.
Morrison has referred to Burisma as a "bucket of issues" — the Bidens, Democrats, investigations — he had tried to "stay away" from.
Sondland met with a Zelenskiy aide on the sidelines of a Sept. 1 gathering in Warsaw, and Morrison, who was watching the encounter from across the room, testified that the ambassador told him moments later he pushed the Ukrainian for the Burisma investigation as a way for Ukraine to gain access to the military funds.
Volker provided investigators with a package of text messages with Sondland and another diplomat, William Taylor, the charge d'affaires in Ukraine, who grew alarmed at the linkage of the investigations to the aid.
Taylor, who testified publicly last week, called that "crazy."
A wealthy hotelier who donated $1 million to Trump's inauguration, Sondland is the only person interviewed to date who had direct conversations with the president about the Ukraine situation.
Morrison said Sondland and Trump had spoken about five times between July 15 and Sept. 11 — the weeks that $391 million in U.S. assistance was withheld from Ukraine before it was released.
Trump has said he barely knows Sondland.
The committee will also hear on Wednesday from Laura Cooper, a deputy assistant secretary of defense, and David Hale, a State Department official. On Thursday, David Holmes, a State Department official in Kyiv, and Fiona Hill, a former top NSC staff member for Europe and Russia, will appear.