The White House attacked a key witness in the House impeachment investigation during his testimony, seeking to undermine one of its own employees as part of a scorched-earth campaign being pursued by allies of President Trump.

Lt. Col. Alexander Vindman, a senior National Security Council official who testified before lawmakers Tuesday, faced withering criticism from Trump’s defenders, who questioned his loyalty to the country and his allegiance to the president. As Vindman spoke, the official White House Twitter account posted a tweet questioning his judgment.

“Tim Morrison, Alexander Vindman’s former boss, testified in his deposition that he had concerns about Vindman’s judgment,” the tweet read. Morrison is a former senior NSC official.

The attacks were an attempt to undercut the testimony from Vindman, a decorated combat veteran who told the House Intelligence Committee that a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky that is at the heart of the impeachment probe was “a partisan play” that prompted him to report it to an NSC lawyer.

Vindman also testified that some of Trump’s allies sought to condition a White House meeting for Zelensky on political investigations targeting former vice president Joe Biden and his son Hunter.

His testimony undercut claims from Trump that his call with Zelensky was “perfect” and that he had done nothing wrong in withholding military aid and a White House meeting from Ukraine.

Vindman defended himself from claims that Morrison, his boss, had expressed concerns about his judgment, reading from his latest performance evaluation in which another former top NSC official described him as “brilliant” and “unflappable.”

On Tuesday, Trump also sought to downplay Vindman’s role and influence.

“I never heard of him. I don’t know any of these people,” Trump told reporters during a Cabinet meeting at the White House. “I don’t know Vindman at all. What I do know is that even he said the transcript was correct.”

While Trump said he would “let people make their own determination” of Vindman, Donald Trump Jr. spent much of Tuesday attacking the Iraq War veteran on Twitter.

“He’s a low level partisan bureaucrat and nothing more,” the younger Trump wrote.

In another post, Trump Jr. wrote, “Seems fair,” in response to a charge that Vindman should face an investigation for perjury.

In a statement after the hearing, which also included testimony from vice presidential adviser Jennifer Williams, White House press secretary Stephanie Grisham defended Trump and decried the impeachment inquiry as “illegitimate.” She also implicitly criticized Vindman and Williams for sharing their “personal opinions and conjecture about a call the White House long ago released to the public.”

On Sunday, Trump attacked Williams as a “Never Trumper” and accused her of trying to attack the president.

Rep. Jim Himes (D-Conn.) asked Vindman and Williams whether they had political reasons for testifying in the impeachment inquiry and whether they would describe themselves as “never Trumpers.”

Both said no.

“Representative, I would call myself ‘never partisan,’” Vindman said.

In his opening statement, Vindman preemptively defended himself from some attacks, emphasizing his up-by-the-bootstraps American story: His family fled the Soviet Union four decades ago to start a new life in the United States, an experience that he said built in him a sense of dedication to his new country. Today, he and his twin brother both serve in the U.S. military are both assigned to the NSC at the White House.

After stating that previous “vile character attacks” on other witnesses in the impeachment inquiry were “reprehensible,” Vindman addressed part of the statement to his father.

“Do not worry, I will be fine for telling the truth,” he said.

Tom Hamburger, Elise Viebeck and John Wagner contributed to this report.