Vindman, 45, was fired from his position on the National Security Council at the White House in February, two days after Trump was acquitted by the Republican-majority Senate.

Vindman was up for promotion to full colonel, but his attorney, David Pressman, said in a statement that would clearly not be forthcoming.

Vindman is "retiring today after it has been made clear that his future within the institution he has dutifully served will forever be limited," Pressman said.

"LTC Vindman's patriotism has cost him his career," he said.

"Through a campaign of bullying, intimidation, and retaliation, the president of the United States attempted to force LTC Vindman to choose: Between adhering to the law or pleasing a President. Between honoring his oath or protecting his career.

"These are choices that no one in the United States should confront, especially one who has dedicated his life to serving it," Pressman said.

Democratic Senator Tammy Duckworth -- a combat veteran who is on the shortlist of candidates to be Joe Biden's presidential running mate against Trump in the fall -- said the resignation "puts the spotlight on Secretary of Defense Mark Esper's failure to protect a decorated combat Veteran against a vindictive Commander in Chief."

She said she would continue to block Senate confirmation of 1,123 senior US Armed Forces promotions until Esper could give a "transparent accounting" of the situation.

Meanwhile, House intelligence chief Adam Schiff released a letter he sent to Vindman thanking him for his service.

"You should not have been subjected to bullying and retaliation from this President. You should not have had to choose between your oath of office and your career. You followed your patriotic and legal duty to tell the truth," the Democrat wrote.

Vindman was present during the July 25, 2019 phone call during which Trump asked Ukrainian President Volodymyr Zelensky to open an investigation into Biden.

Subpoenaed by Congress to testify at the House impeachment hearings, the Ukrainian-born Vindman, who received a Purple Heart for wounds suffered in Iraq, said Trump's actions were "improper."

That testimony helped build the case leading to Trump becoming only the third president ever impeached by Congress.

Vindman's twin brother, a lawyer on the National Security Council, was not called to publicly testify but was also forced out of his White House position in February.

John Bolton, who served as Trump's National Security Advisor when Vindman was at the White House, told MSNBC television that his performance had been "exceptional."

"I think he merited promotion," said Bolton, who recently published a tell-all book about his time in the Trump White House that is harshly critical of the president.

"And I'm sure that Congress is going to be very interested over the coming weeks as to exactly what factored into his decision," he said.