The Senate’s top Democrat is urging military officials to protect the reputation and safety of an Army officer turned key witness in the ongoing impeachment investigation into President Donald Trump.

Lt. Col. Alexander Vindman, a 20-year active-duty soldier now assigned to the National Security Council as a Ukraine expert, was subpoenaed by House lawmakers earlier this week to testify about a disputed July 25 call between Trump and Ukraine President Volodymyr Zelenskiy.

In testimony released before his appearance, Vindman said he had concerns that Trump appeared to “demand that a foreign government investigate a U.S. citizen," referring to allegations leveled against family members of Democratic presidential rival Joe Biden.

Since his appearance, Vindman background in the military and as a Ukrainian immigrant has received public scrutiny, including from the president himself. On Wednesday, he referred to Vindman on Twitter as a “Never Trumper witness” and accused him of being part of a “witch hunt.”

Later in the day, Senate Minority Leader Chuck Schumer, D-N.Y., implored Army officials to defend the soldier.

“He is a patriot for being willing to do what we hope and expect every service member will do: to tell the truth when asked,” Schumer said in a letter to Army Secretary Ryan McCarthy and Army Chief of Staff Gen. James McConville. “It is incumbent on the Army to ensure that he is afforded the same protections as whistleblowers and protected from reprisal for testifying before Congress.”

The letter follows a similar one Schumer sent to intelligence officials requesting additional protections for whistleblowers testifying before Congress.

In the case of Vindman, Schumer noted that “although he has served our country for more than 20 years and is a recipient of the Purple Heart after being injured while serving in Iraq, he has been called a variety of derogatory terms and some have even gone so far as to call him a spy and question his loyalty to the United States.”

He called the attacks “outrageous and unacceptable” and warned that “this vitriol toward LTC Vindman may result in professional reprisals and threats to his personal safety and that of his family.”

Several Republican lawmakers have also condemned the criticism, including Wyoming Rep. and House Republican Conference Chair Liz Cheney, who called the attacks on Vindman shameful.

“We need to show that we are better than that as a nation,” Cheney said. “We’re talking about decorated veterans who have served this nation, who have put their lives on the line. It is shameful to question their patriotism, their love of this nation, and we should not be involved in that process."

In his testimony, Vindman said that he did express concerns about the Trump call “internally to National Security officials” but denied being the whistleblower who publicly brought forth potential criminal actions related to the conversation.