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(CNN) - Lt. Col. Alexander Vindman, an adviser to the National Security Council who has been criticized by President Donald Trump and his allies, told lawmakers on Tuesday that he reported concerns about Trump's call on July 25 with the president of Ukraine, who is at the center of the political trial investigation, for a "sense of duty." He also defended the other witnesses to the political trial of what he described as "reprehensible" attacks.
Vindman, who testified with his Army uniform as an active duty military, invoked his father's decision to leave the Soviet Union and come to the United States, arguing that in Russia they could kill him for the testimony he gave Tuesday.
"Don't worry, nothing will happen to tell the truth," said Vindman.
The military rejected attacks against his colleagues who have appeared during the political trial investigation, saying that "it is reprehensible that the character of these distinguished and honorable public servants is attacked."
"It is natural to disagree and participate in a heated debate, it has been that way since the time of our Founding Fathers, but we do not need to fall into personal attack," he said.
Vindman testified that he informed the National Security Council's lawyers of his concerns through the appropriate channels on the July 25 call and a July 10 meeting at which a US ambassador proposed to open investigations to the Ukrainians present . He said he raised these concerns in private "because they had important national security implications for our country."
The military said it was "inappropriate" and "improper" for the president to demand an investigation into a political opponent.
Vindman testified alongside Jennifer Williams, an adviser to Vice President Mike Pence, who like Vindman also expressed concern over the July call and was attacked by the president after testifying. Williams said in his opening statement that he considered the call to be "unusual" because it included "a discussion about what appeared to be an internal political issue." But he clarified that he did not discuss the call with the vice president.
Tuesday's hearing begins a marathon week in which political trial investigators will publicly listen to nine witnesses in five hearings over the next three days, in an attempt by Democrats to gather public testimony about President Donald Trump's role in pressuring Ukraine to open investigations and retention of United States security assistance.
On Tuesday afternoon, the former US special envoy to Ukraine, Kurt Volker, and Tim Morrison, former senior adviser to Russia and Europe on the National Security Council, will testify.
(With information from Lauren Fox of CNN, Kristen Holmes, Pamela Brown, Phil Mattingly and Kaitlan Collins)