A security staffer who listened to the call between President Donald Trump and Ukraine says he was concerned by the demands for a probe into a Trump rival.
Alexander Vindman, a decorated army veteran, told Congress he reported his objections to pressuring Ukraine to investigate Democrat Joe Biden twice.
Col Vindman is the first White House official who heard the call to testify as a part of the impeachment inquiry.
The inquiry concerns alleged abuse of power by the president.
Col Vindman was among select officials who listened in on his 25 July call with newly elected Ukrainian President Volodymyr Zelensky, which sparked a whistleblower complaint and led to the impeachment probe.
Mr Trump is accused of trying to pressure Ukraine into investigating discredited corruption claims against Mr Biden and his son, who worked with Ukrainian gas company Burisma while his father was the US vice-president.
Mr Trump has denied wrongdoing and called the impeachment inquiry a “witch hunt”.
Though the White House has called on witnesses to disregard congressional subpoenas, as an active-duty military officer, Col Vindman could have faced sanction for doing so.
What did Col Vindman say?
In an opening statement released ahead of Tuesday’s testimony, Col Vindman says his worries began at a 10 July meeting between US and Ukrainian national security officials.
The meeting was cut short by then-National Security Advisor John Bolton when talk of Ukraine opening investigations for the White House arose, Col Vindman said.
“The meeting proceeded well until the Ukrainians broached the subject of a meeting between the two presidents,” Col Vindman said, noting that Ukraine saw this as “critically important” for maintaining US support.
US Ambassador Gordon Sondland then brought up “Ukraine delivering specific investigations in order to secure the meeting with the president” which alarmed Mr Bolton and Col Vindman.
At a debriefing afterwards, Col Vindman said the ambassador again “emphasised the importance that Ukraine deliver the investigations into the 2016 election, the Bidens and Burisma”.
“I stated to Amb Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security.”
Following this incident, Col Vindman reported his concerns to the National Security Council’s lead counsel. He reported his objections again after the 25 July call.
“I did not think it was proper to demand that a foreign government investigate a US citizen,” Col Vindman says in the statement. “I was worried about the implications for the US government’s support of Ukraine.”
He said having that country look into the Bidens would “undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained”.
“This would all undermine US national security,” he added.
What’s the reaction?
As the testimony was due to begin, Mr Trump suggested Col Vindman was a “Never Trumper witness” in a tweet.
Other conservatives have also attacked Col Vindman’s credibility because he was born in Ukraine – though some have since defended the veteran officer.
Republican congresswoman Liz Cheney denounced these criticisms as “shameful” at a news conference with other House Republican leaders.
We are talking about decorated veterans who have put their lives on the line,” she said.
But some Republicans have also continued to defend him. House Minority leader Kevin McCarthy said Col Vindman was “wrong”.
“Nothing in that phone call is impeachable,” he said.
Who is Col Vindman?
In 2018, Col Vindman joined the security council under former National Security Advisor John Bolton, who was fired by Mr Trump earlier this year.
His family fled the Soviet Union in 1979, when he was three years old. He has served in the military for two decades as an active officer and diplomat.
As an infantry officer, he was deployed to Iraq – where he was wounded and received the Purple Heart military award.
Col Vindman later served at US embassies in Ukraine and Russia since 2008.