Impeachment witness has message for his father: 'I will be fine for telling the truth.'
Vindman also said in his opening statement: "It is improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent."
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.
