WASHINGTON         -          A White House official testified in the impeachment inquiry against President Donald Trump Tuesday that the president’s phone call urging Ukraine’s leader to investigate Trump’s political rivals was improper and blasted what he called “cowardly” attacks on witnesses in the investigation.

Army Lieutenant Colonel Alexander Vindman, the White House National Security Council’s top Ukraine expert, testified at the third public hearing in the impeachment effort before the House of Representatives Intelligence Committee.

Vindman, who along with other witnesses has been publicly criticized by Trump, also told lawmakers that “vile character attacks” against public servants testifying in the impeachment inquiry were “reprehensible,” urging Americans to be “better than callow and cowardly attacks.” Vindman, an Iraq war veteran, appeared at the hearing wearing his Army uniform and medals.

Vindman did not specifically mention Trump when he referred to “cowardly attacks.” Some Trump allies in the conservative media have questioned Vindman’s loyalty to the United States.

“It is improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent,” Vindman said in his opening statement in the second week of public hearings.

Three other witnesses were scheduled to testify at Tuesday’s hearing: Kurt Volker, the former U.S. special envoy to Ukraine; Jennifer Williams, an aide to Vice President Mike Pence; and former National Security Council Russia expert Tim Morrison.

The inquiry focuses on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to carry out two investigations that would benefit him politically including one targeting Democratic political rival Joe Biden. The other involved a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 U.S. election.

In her prepared statement to the committee, Williams said Trump’s July 25 call was “unusual” because it “it involved discussion of what appeared to be a domestic political matter.” Williams said the White House Budget office had said Trump’s acting chief of staff, Mick Mulvaney, had directed that $391 million in security aid to Ukraine be put on hold and that she never learned why the assistance was later released in September.

Williams, who was attacked by Trump on Twitter just days before her public appearance, also told lawmakers that she was committed to serving America’s interests, adding “it was with great pride and conviction that I swore an oath to uphold and defend the Constitution.”

Williams was among the U.S. officials who monitored the Trump-Zelenskiy call. She previously testified behind closed doors this month that some of Trump’s comments were “inappropriate.”

Trump assailed her on Twitter on Sunday as a “Never Trumper” who should “work out a better presidential attack.”