WASHINGTON - U.S. Senate Democratic leader Chuck Schumer asked the Defense Department watchdog on Monday to investigate what he said was "shameful" and "Illegal" retaliation by President Donald Trump against a witness who testified against him at last year's impeachment hearings.

Schumer asked 74 inspectors general throughout the government to probe any retaliation against federal workers who report Trump administration misconduct, but singled out the president's dismissal of Army Lt. Col. Alexander Vindman as a Ukraine expert at the National Security Council in the White House. Vindman testified against Trump at House of Representatives impeachment hearings leading up to approval of two articles of impeachment, although Trump last week was acquitted after a three-week trial in the Senate.

A short time before dismissing Vindman, Trump was asked about his status, responding, “I’m not happy with him. You think I’m supposed to be happy with him? I’m not."

Vindman testified about concerns he had about Trump's request last July to Ukrainian President Volodymyr Zelenskiy to investigate one of Trump's chief 2020 Democratic challengers, former Vice President Joe Biden, his son Hunter Biden's lucrative work for a Ukrainian natural gas company and a debunked theory that Ukraine meddled in the 2016 U.S. election to undermine Trump's campaign. Trump's request came at the same time he was temporarily blocking release of $391 million in military aid Kyiv wanted to help fight pro-Russian separatists in eastern Ukraine.

FILE - Lt. Col. Alexander Vindman, left, walks with his twin brother, Yevgeny Vindman, after testifying before the House Intelligence Committee, on Capitol Hill in Washington, Nov. 19, 2019.

In addition, Trump ousted Vindman's twin brother, Yevgeny (Eugene) Vindman, also an Army lieutenant colonel and an ethics attorney at the security council, returning both of them to positions at the Defense Department. Within hours on Friday, Trump also fired Gordon Sondland, who had donated $1 million to Trump's 2017 inauguration, as the U.S. ambassador to the European Union in Brussels. Sondland angered Trump by testifying that the president had engaged in a quid pro quo with Ukraine, withholding a White House meeting that Zelenskiy wanted until he announced the Biden investigations.

Trump, after a 55-day delay, released the military aid in September without Zelenskiy announcing any Biden investigations.

Schumer told Defense Department acting inspector general Glenn Fine that Alexander Vindman "lived up to his oath to protect and defend our Constitution by bravely stepping forward to tell the truth." But Schumer said Vindman "has been viciously attacked by the president and forced to endure threats to his and his family's safety."

Schumer, leader of the minority bloc of Senate Democrats, said, "These attacks are part of a dangerous, growing pattern of retaliation against those who report wrongdoing only to find themselves targeted by the president and subject to his wrath and vindictiveness. They also include attempts to publicly identify the anonymous whistleblower who used the proper legal channels to initially report the president's attempts to compromise our national security for his personal benefit."

The lawmaker urged Fine to "take immediate action to investigate any and all instances of retaliation against anyone who has made, or in the future makes, protected disclosures of presidential misconduct to Congress or inspectors general." Schumer also demanded that the inspectors general throughout the government notify Congress of the last time that federal workers at their agencies were reminded of their legal rights to "make protected disclosures anonymously and free from reprisals."

The Senate cleared Trump of both impeachment charges against him, that he abused the power of his presidency and obstructed congressional efforts to investigate his Ukraine-related actions.