Lt Col Alexander Vindman, a military veteran with 20 years' experience, was so troubled by what he heard that he promptly contacted a White House lawyer, a hearing on Capitol Hill was told.
“Without hesitation, I knew that I had to report this to the White House counsel,” said Lt Col Vindman.
Jennifer Williams, a special adviser on Europe and Russia to vice president Mike Pence, said she found the call “unusual” because it related to domestic politics, but she did not report it to anyone
The pair testified before the House intelligence committee on Tuesday, as the latest witnesses subpoenaed by Democrats to provide public evidence to their impeachment inquiry into what they have termed Mr Trump’s “bribery” in regard to Ukraine.
Ahead of Tuesday’s session, it was stressed the information the witnesses possessed was of particular importance because unlike some of those called to give evidence, the pair both listened in on Mr Trump’s July 25 phone call with Volodymyr Zelensky.
Lt Col Vindman, 44, a veteran of the Iraq war and director for European affairs at the national security council, said he had immediately passed on his concerns to John Eisenberg, a White House lawyer.
“It was inappropriate for the president to request – to demand – an investigation into a political opponent, especially [from] a foreign power where there is at best dubious belief that this would be a completely impartial investigation and that this would have significant implications if it became public knowledge,” the officer told the committee.
Lt Col Vindman also condemned Mr Trump’s Twitter attack on a previous witness.
He said: “I want to state that the vile character attacks on these distinguished and honourable public servants is reprehensible. It is natural to disagree and engage in spirited debate, this has been our custom since the time of our Founding Fathers, but we are better than callow and cowardly attacks.”
He spoke after Mr Trump last week used social media to attack Marie Yovanovitch, a former US ambassador to Ukraine, who testified about how US policy on Ukraine had been upended by Mr Trump and his personal lawyer, Rudy Giuliani.
Lt Col Vindman, who who appeared wearing his army uniform and medals, and who at one point asked he be addressed by his rank, told the committee his family fled the Soviet Union and settled in the United States.
“Dad, that I am sitting here today, in the US Capitol talking to our elected officials is proof that you made the right decision 40 years ago to leave the Soviet Union and come here to the United States of America in search of a better life for our family. Do not worry, I will be fine for telling the truth,” he said.
He said felt strong enough to strand up to the most powerful person in the world, because “this is America”.
Ms Williams, who has served in Jamaica, Lebanon and the United Kingdom, said Mr Trump’s call with Zelensky was unusual because “it involved discussion of what appeared to be a domestic political matter.”.
She said the White House budget office had said Mr Trump’s acting chief of staff, Mick Mulvaney, had directed $391m in security aid to Ukraine be put on hold and that she never learned why the assistance was later released in September.
Democrats launched an impeachment investigation after a whistleblower, believed to be a member from the US intelligence community, alleged that in his call to Mr Zelensky, Mr Trump had requested he launch an investigation into Joe Biden and his son, in exchange for the release of military aid and a state visit to the US.
Mr Trump, who has denied any wrongdoing, has described the investigation as a “witch hunt”. On Tuesday, he said he did not know the officer and mocked him for wearing his military uniform.
Democrats have been accused of intentionally changing the language they use to describe the proceedings after discovering from focus groups that “bribery” had more traction with voters than the phrase “quid pro quo”.
Adam Schiff, the Democratic chair of the committee, defended use of the word.
“The fact that they failed in this solicitation of bribery doesn’t make it any less bribery, doesn’t make it any less immoral or corrupt,” he said. “It just means it wasn’t successful.”
Congressman Devin Nunes, the committee’s top Republican and a close ally of the president, did not address the substance of the witness testimony about the president’s actions in his opening statement, but unleashed an attack on the media, calling journalists “puppets of the Democratic Party”.
Additional reporting by Reuters