His ouster, which followed by days Trump's acquittal in the Senate after being impeached by the House, was widely seen as retribution for having told a House panel in October that he had been alarmed by a July phone call he listened in on between Trump and Ukrainian President Volodymyr Zelenskiy. Trump had pressured Zelenskiy in that call to investigate former Vice President Joe Biden, who at the time was a leading Democratic contender to challenge Trump for the presidency.

Vindman's twin brother, Yevgeny, was fired the same day from his job as an ethics lawyer on the National Security Council. He had not testified in the House impeachment inquiry.

Vindman had been expected to be on a list of Army lieutenant colonels due for promotion this year. But the Pentagon has yet to release that list, which would then be subject to review by the White House as well as the Senate.

Hours before the Vindman brothers were fired, Trump was asked by a reporter if he wanted Alexander Vindman out of the White House.

"Well, I'm not happy with him," Trump replied. "Do you think I'm supposed to be happy with him? I'm not."

The following day, Trump wrote in a tweet that he did not believe he had ever met Vindman, but nonetheless described the ousted officer as "very insubordinate, reported contents of my 'perfect calls' incorrectly," and added, "In other words, 'OUT'."

There has been growing consternation at the Pentagon and on Capitol Hill about the fate of Vindman, who immigrated from Ukraine as a child and was awarded the Purple Heart for his service in the Iraq war.

Without referring to Vindman by name, Rhode Island Sen. Jack Reed, the Senate Armed Services Committee's ranking Democrat, issued a statement June 18 affirming that elected officials should not interfere with the military's merit-based promotion system.

"The promotion process is about individuals, but it also sends a signal to our troops about what kind of soldiers get promoted," Reed wrote. "Professionalism, character, skill, and integrity must be paramount, not partisanship."

Now Duckworth, a former Illinois National Guard Blackhawk helicopter pilot who lost both legs fighting in Iraq, is taking action that her office calls "unprecedented in modern times" to force a resolution to the promotion holdup, which she ties to Trump.

"It is simply unprecedented and wrong for any Commander in Chief to meddle in routine military matters at all, whether or not he has a personal vendetta against a Soldier who did his patriotic duty and told the truth—a Soldier who has been recommended for promotion by his superiors because of his performance," Duckworth says in a statement. "I won't just sit by and let it happen, and neither should any of my colleagues."

Duckworth is making one exception in her sweeping hold to allow Senate confirmation of Army Gen. Gustave Perna to head the effort to develop a vaccine to protect against COVID-19.

The Illinois senator, who is a Purple Heart recipient, lists three conditions for lifting her hold on any other military nominations:

--She wants the defense secretary to provide written confirmation that Vindman is among the lieutenant colonels who were considered for promotion by the Army's promotion board.

--If he was, she wants to know if he was on the list for promotions the Army submitted to the Pentagon's personnel office.

--And finally, Duckworth wants Esper to inform her whether a list for promotions that includes Vindman has been or will be sent to the White House for approval.

A failure to be promoted would normally spell the end of a military career.