(CNN) A top Pentagon official as recently as last month vowed the department would not tolerate "any act of retaliation" against Lt. Col. Alexander Vindman -- a pledge now being put to the test after Republican Sen. Marsha Blackburn this week repeatedly attacked the key National Security Council aide who testified before Congress on the Ukraine scandal.

Responding to concerns about Vindman's safety following his November testimony in the House impeachment inquiry, Deputy Defense Secretary David Norquist -- on Defense Secretary Mark Esper's behalf -- assured Senate Minority Leader Chuck Schumer that the department would take any "steps necessary" to ensure the White House detailee's safety, according to a copy of a December 16 letter to Schumer that was obtained by CNN.

"Let me assure you, the Department will not tolerate any act of retaliation or reprisal," Norquist wrote to Schumer, noting that he was speaking on behalf of his boss, Esper.

"Further, we are committed to taking whatever steps are necessary, consistent with Section 714 of Title 10, US code, to ensure the security of LTC Vindman," he added. Norquist also promised to provide the same protections for Deputy Assistant Secretary of Defense Laura Cooper, another impeachment witness who provided key testimony in the House probe.

The portion of US code cited by Norquist primarily relates to the Defense Secretary's authority over matters involving individuals within the chain of command. Still, the spirit of that letter has taken on new relevance after Blackburn, a Tennessee Republican and an ardent defender of President Donald Trump, spent hours attacking Vindman on Twitter Thursday as Democratic impeachment managers made the House's opening statements in the President's Senate impeachment trial

In a series of tweets, she questioned the Purple Heart recipient's patriotism and repeated the unfounded claim that he had leaked knowledge of Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky to the whistleblower whose complaint initially prompted the investigation into the President's conduct in Ukraine.

"Adam Schiff is hailing Alexander Vindman as an American patriot," Blackburn said in a tweet at about 4 p.m. ET, on Thursday. "How patriotic is it to badmouth and ridicule our great nation in front of Russia, America's greatest enemy?"

Later, at about 6 p.m. ET, she claimed without evidence that Vindman leaked the July phone call to the whistleblower, adding: "They both have lots in common."

CNN is not identifying the whistleblower, but there is no evidence that Vindman or the whistleblower knew each other or held any political motivations.

Blackburn accused both individuals of having "held the same NSC job," being "liberals who worked under Obama" and having "wanted to take out Trump."

It is unclear whether Blackburn or her staff sent the tweet as it posted during oral arguments in the Senate impeachment trial , which bans members from using electronics. She tweeted again about Vindman at about 8 p.m. ET, on Thursday.

As the President's lawyers prepare to begin their own arguments, Schumer is calling on Esper to publicly defend Vindman and condemn Blackburn's remarks.

"Lieutenant Colonel Alexander Vindman is a patriot. He does not deserve these disgraceful attacks for standing up and defending the Constitution," Schumer said in a statement to CNN. "The Department of Defense must live up to what they told me, stand up for Lt. Col. Vindman, and protect him from reprisals. Secretary Esper should condemn Senator Blackburn's comments."

CNN has reached out to the Pentagon for reaction to the letter.

Pentagon spokesman Jonathan Hoffman demurred when asked Friday if it was appropriate for a member of Congress to question the patriotism of an active duty service member.

"I'm not going to wade into the hearings and things that are taking place on Capitol Hill right now," he told CNN, referring to the ongoing Senate impeachment trial.

"Obviously it's ... a highly charged environment so I'm going to not weigh in on that other than to say that Colonel Vindman is still an active duty member of the military and continuing to perform his job on a daily basis," he said.

While testifying before the House panel in November, Vindman told lawmakers that Trump's push for Ukraine to investigate former Vice President Joe Biden on a July call was "inappropriate," and he knew "without hesitation" that he had to report it, prompting personal attacks from the President and his closest supporters.

At the time, Vindman reached out to the US Army about the security of his family but a defense official told CNN that the service did not believe there was an imminent security threat against the decorated veteran.

Blackburn's public attacks against Vindman fueled criticism of the senator, particularly over a tweet she posted saying: "Vindman's commanding officer, Army Lt. Col. Jim Hickman: 'Do not let the uniform fool you. He is a political activist in uniform.'"

Retired Brig. Gen. Peter Zwack, who says he was Vindman's commanding officer between 2012 and 2014, pushed back on Hickman's assertions and credibility on Twitter Friday following Blackburn's tweet.

"Dear Sen Blackburn. I was LTC Vindman's boss in Moscow betw 2012-14. LTC Hickman was not. Shall I visit you to privately discuss his loyalty & reliability w/ you. Pls take these unfounded, corrosive accusations out of the public domain. They're wrong & hurtful!" he said in the tweet.

David Pressman -- Vindman's attorney and a former ambassador to the United Nations for special affairs -- has also slammed Blackburn's tweets in a statement.

"This difficult moment in our country calls for seriousness and seriousness of purpose. Lieutenant Colonel Alexander Vindman has sacrificed enormously for our country. He believes in our country. And he believes in our country's great institutions, including the United States Senate," Pressman said.

"That a member of the Senate -- at a moment when the Senate is undertaking its most solemn responsibility -- would choose to take to Twitter to spread slander about a member of the military is a testament to cowardice," he continued. "While Senator Blackburn fires off defamatory tweets, Lieutenant Colonel Vindman will continue to do what he has always done: serve our country dutifully and with honor."