President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's aggressive attacks on a White House official who testified about his concerns over Trump's communications with Ukraine in the impeachment inquiry set off a furious backlash on Tuesday, with former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE calling the president's remarks “despicable.”

Trump described Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, an active-duty member of the military who attended his deposition in uniform, as a “Never Trumper,” while some allies questioned the Purple Heart recipient’s patriotism given the fact that he emigrated from Ukraine as a child.

Sen. Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-Utah) called the attacks “absurd,” while Rep. Liz Cheney Elizabeth (Liz) Lynn CheneyGOP lawmakers distance themselves from Trump comments on transfer of power Graham vows GOP will accept election results after Trump comments Liz Cheney promises peaceful transfer of power: 'Fundamental to the survival of our Republic' MORE (Wyo.), the No. 3 Republican in the House, admonished those who would question Vindman’s patriotism without naming names.

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“Their patriotism, their love of country — we're talking about decorated veterans who have served this nation, who put their lives on the line, and it is shameful to question their patriotism, their loving this nation, and we should not be involved in that process,” Cheney said at the House GOP leadership’s press conference.

Trump in a Tuesday tweet criticized Vindman as he prepared to testify before three congressional committees behind closed doors, suggesting the official was participating in a “witch hunt” against him.

“Supposedly, according to the Corrupt Media, the Ukraine call ‘concerned’ today’s Never Trumper witness,” Trump tweeted. “Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt!”

The attack was on-brand for Trump, who has a well-earned reputation as a counterpuncher when criticized. Just last week, Trump slammed Vietnam veteran and U.S. diplomat William Taylor, using the same “Never Trumper” insult after decrying such critics as “human scum.”

Trump has previously targeted critics with military ties and bipartisan respect, including the late Sen. John McCain John Sidney McCainCrenshaw looms large as Democrats look to flip Texas House seat Analysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture MORE (R-Ariz.) and Khizr Khan, the Gold Star father who spoke at the 2016 Democratic National Convention.

But the attacks on Vindman, coupled with comments from allies such as former Rep. Sean Duffy Sean DuffyCNN's Ana Navarro to host Biden roundtable on making 'Trump a one-term president' Bottom line McCarthy blasts Pelosi's comments on Trump's weight MORE (R-Wis.), ran into opposition from Republicans and highlighted a GOP divide even as the party seeks to shift its strategy on fighting Democrats over Trump's impeachment.

Vindman on Tuesday provided damaging testimony about Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky.

Vindman, a firsthand witness to the phone call, told lawmakers that he was so concerned about Trump raising an investigation into Biden, a 2020 Democratic presidential candidate, and his son Hunter Biden’s business dealings in Ukraine that he reported it to the National Security Council’s (NSC) lawyer, according to a copy of his prepared remarks that leaked out Monday night.

“I realized that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine U.S. national security,” his remarks read.

Vindman, a Ukrainian refugee who fled to the United States with his family at age 3, is a former infantry officer who has served for two decades in the U.S. Army. He has deployed overseas multiple times, including serving a tour in Iraq where he was wounded by an improvised explosive device and received a Purple Heart. He has been an Army foreign area officer since 2008 and was asked to join Trump’s NSC in July 2018.

Vindman serves as the NSC’s director of European affairs, a role one former NSC official described as apolitical and typically filled with detailees from other parts of the federal government.

“It’s an apolitical role,” said the former official. “You’re not part of the policy team in terms of coming up with policy, you’re enacting policy.”

One former Trump administration official said the barrage of attacks on witnesses like Vindman, Taylor and former U.S. Ambassador to Ukraine Marie Yovanovitch can be demoralizing for other career officials still in government.

The barbs may not abate for some time, with a cast of current and former officials scheduled to testify.

Vindman’s Tuesday testimony could prove problematic for the White House, particularly as Trump has urged his allies to focus on substance over process in defending him despite some reluctance from Republican lawmakers.

“I thank him for his service. I thank him for his commitment to this country, but he is wrong,” House Minority Leader Kevin McCarthy Kevin Owen McCarthyHouse to vote on resolution affirming peaceful transition of power Ginsburg becomes the first woman to lie in state in the Capitol McCarthy says there will be a peaceful transition if Biden wins MORE (R-Calif.) said of Vindman during a press conference.

Other Republican lawmakers deflected Vindman’s testimony by saying they felt the conduct outlined in the July 25 call was not impeachable.

But some of the president’s allies suggested Vindman’s Ukrainian roots might cloud his judgment.

“It seems very clear that he is incredibly concerned about Ukrainian defense,” said Duffy, the former congressman who is now a CNN commentator. “I don't know that he's concerned about American policy, but his main mission was to make sure the Ukraine got those weapons. I understand that we all have an affinity to our homeland where we came from.”

Trump’s personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE claimed that Vindman felt “confused” because he was “advising two [governments].”

Republican strategists dismissed the impact of Trump’s attacks, arguing Republicans may disagree with Trump over them but it wouldn’t compromise the president’s support amid the impeachment inquiry.

“Republicans who oppose that strategy will feel no compunction vocalizing that,” said one Republican strategist, citing Cheney’s remarks. “It doesn’t change the reality that he’s not going to be convicted in the Senate and impeachment is a waste of time.”

GOP strategist Ford O’Connell also said Trump is “right to challenge the veracity of witnesses coming before Congress,” noting that Vindman was presenting his opinion of Trump’s phone call with Zelensky and that the partial transcript released by the White House did not show an explicit quid pro quo on the phone call.

Still, it could complicate efforts to unify the White House and Republicans on messaging to effectively fend off the fast-moving impeachment inquiry.

Trump’s attacks come as the White House is struggling to adopt a coherent strategy to counter the growing body of evidence of impropriety in Trump’s interactions with Ukraine. One former White House official argued attacking witnesses wouldn’t serve the purpose of a strategy unless the attacks were credible.

“They have to rebut the claims and find credible angles of attack. Just blowtorching them isn’t a strategy,” said the former official.

“Overall, they should take a page from the Clinton playbook. Democrats are focused on tearing the president down instead of lifting the American people up; Democrats aren’t focused on your hopes and dreams, they’re focused on their own political ambition,” the former official continued.

House Democrats will vote on a resolution this week laying out the next steps for the investigation, with plans to take hearings with witnesses public over the next few weeks. The vote will force lawmakers to take a public position on the impeachment inquiry, which could work in the White House’s favor by putting vulnerable Democrats in a tough position.

The White House has refused to cooperate with the inquiry, casting it as “illegitimate” because Democrats refused to subject it to a formal vote, but officials have not committed to cooperating once a vote takes place.

Jordain Carney contributed.