A retired US Army lieutenant-colonel who was a pivotal witness in the first impeachment case against Donald Trump has sued the oldest son of the former president and other Trump allies, accusing them of participating in an "intentional, concerted campaign of unlawful intimidation and retaliation" over his decision to testify. 

The lawsuit from Alexander Vindman, who testified during the 2019 impeachment proceedings about a phone call in which Trump pressed his Ukraine counterpart to investigate Democratic rival Joe Biden and his son, Hunter, was filed today in US federal court in Washington. Trump was impeached by the House but acquitted by the Senate in February 2020. 

The suit names as defendants: Donald Trump Jr; Rudy Giuliani, a long-time Trump adviser who has served as Trump's lawyer; and former White House communications officials Dan Scavino and Julia Hahn. 

It alleges that after Vindman was summoned by House lawmakers to testify, the defendants and others co-ordinated and advanced "false narratives" about him, including that he was a spy for Ukraine and had spoken pejoratively about the US to foreign officials; leaked classified information about him; falsely accused him of lying under oath; and worked to derail his expected promotion to colonel. 

Alexander Vindman, a military officer at the National Security Council who testified during the impeachment hearings on Capitol Hill. Photo / AP 

"The actions taken by Defendants against Lt Col Vindman sent a message to other potential witnesses as well: co-operate at your own peril," the suit states, adding that "the message reverberates to this day" as witnesses in the congressional investigation into the January 6 riot at the US Capitol defy subpoenas at the Republican former president's direction. 

Lawyers for Trump Jr, Scavino and Giuliani did not immediately return messages seeking comment. Hahn also did not return an email seeking comment. 

Vindman, an Iraq war veteran whose family fled the Soviet Union when he was a child, was a key impeachment witness, testifying that he twice raised concerns over the Trump administration's push to have Ukraine investigate Biden. After listening to the July 2019 call between Trump and Ukraine leader Volodymyr Zelenskyy, he said he reported his concerns to a lawyer at the National Security Council. 

Vindman says in the suit that he swiftly became a "household name" thanks to an intimidation campaign laden with falsehoods. 

"The attacks on Lt Col Vindman did not simply happen by accident or coincidence, nor were they the result of normal politics or modern news cycles," the lawsuit states. 

"Rather," it adds, "the co-ordinated campaign was the result of a common understanding and agreement among and between President Trump, defendants, and others comprising a close group of aides and associates inside and outside of the White House, to target Lt Col Vindman in a specific way for the specific purpose of intimidation and retaliation." 

Donald Trump has been heavily involved in recent political rallies. Photo / Getty Images 

As a result of the treatment, Vindman "has sustained significant financial, emotional, and reputational harm," according to the lawsuit, which seeks compensatory and punitive damages. 

Vindman announced in July 2020 that he was retiring from the US Army after more than 21 years, and at the time raised similar allegations of harassment, bullying and intimidation as in Wednesday's lawsuit. He has since come out with a book about his experiences, titled, Here, Right Matters: An American Story.