Eight weeks into the House impeachment inquiry, President Trump and many of his allies have seized on a core defense strategy by attacking career public servants who are testifying as witnesses in the probe and spreading disinformation about their motives as “unelected bureaucrats.”

The tactic was deployed in a prominent way Monday when Sen. Ron Johnson (R-Wis.) laid out criticisms against Lt. Col. Alexander Vindman, a National Security Council official who is poised to give key public testimony Tuesday. Johnson wrote without evidence that Vindman may be a member of a rebellious “deep state” that “never accepted President Trump as legitimate” and is working in secret to end his presidency.

“I believe a significant number of bureaucrats . . . resent [Trump’s] un­or­tho­dox style and his intrusion on their ‘turf,’ ” Johnson wrote to the top Republicans on the House Oversight and Intelligence committees. “They react by leaking to the press and participating in the ongoing effort to sabotage his policies and, if possible, remove him from office. It is entirely possible that Vindman fits this profile.”

Rep. Adam B. Schiff (D-Calif.) on Nov. 15 said President Trump’s tweet criticizing Ambassador Marie Yovanovitch was witness intimidation. (The Washington Post)

Johnson’s letter intensifies a campaign of attacks on Vindman from Trump and his allies, which has included speculation about the decorated war veteran’s patriotism from conservative commentators and a White House statement on Friday criticizing his job performance. Moves such as these have gained significant traction with Trump’s base, feeding into an echo chamber that stokes supporters’ resentments, broadcasts a single pro-Trump message and demonstrates the power of the online juggernaut Democrats will confront during Trump’s 2020 reelection campaign.

Much of this messaging has taken aim at the career public servants cooperating with the House impeachment inquiry.

Sharing a sentiment on Friday that gained viral popularity among his father’s supporters, Donald Trump Jr. tweeted that “America hired [Trump] to fire people like the first three witnesses we’ve seen.” He was referring to former ambassador to Ukraine Marie Yovanovitch, acting ambassador to Ukraine William B. Taylor Jr. and top State Department official George Kent, all of whom testified last week.

The three are “career government bureaucrats and nothing more,” Trump Jr. wrote.

The rhetoric offers a preview of what could be in store this week, as nine witnesses prepare to testify in open impeachment hearings on Capitol Hill. During Yovanovitch’s five-hour testimony Friday, Trump denigrated her on Twitter, a sign of his readiness to undercut witnesses in real time using social media.

Vindman has offered important information in the investigation of whether Trump sought to pressure Ukraine for political favors by withholding military aid, raising the stakes for Tuesday’s hearing. He will speak alongside Jennifer Williams, a former special adviser on Europe and Russia to Vice President Pence who was the target of a hostile Trump tweet on Sunday in which he labeled her a “never Trumper.”

In a closed-door deposition last month, Vindman gave a firsthand account of the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky, telling lawmakers he was so concerned that he reported the call to the top lawyer for the National Security Council. Williams, who was privately deposed earlier this month, said her contemporaneous notes from the call contrast with the transcript summary released by the White House; specifically, she noted that Zelensky mentioned Burisma, the Ukrainian energy company where Hunter Biden served as a board member.

The central question of the impeachment probe is whether Trump tried to coerce Zelensky to investigate his political rivals, including 2020 presidential candidate and former vice president Joe Biden and his son Hunter.

Witnesses’ cooperation with investigators, in defiance of the Trump administration, has stoked long-standing resentments for the president, who has denigrated civil servants and other longtime government officials as political enemies bent on thwarting his agenda.

On Friday, Yovanovitch was the target of a tweet 30 minutes after she was sworn in.

“Everywhere Marie Yovanovitch went turned bad,” Trump wrote in a message that has been shared more than 32,600 times. “She started off in Somalia, how did that go? Then fast forward to Ukraine, where the new Ukrainian President spoke unfavorably about her in my second phone call with him.”

When asked for her response, Yovanovitch said: “I actually think that where I’ve served over the years, I and others have demonstrably made things better for the U.S. as well as for the countries that I’ve served in.”

That afternoon, the White House blamed Vindman for a discrepancy between the readout of a call between Trump and Zelensky in April and the rough transcript, which was released Friday.

“It is standard operating procedure for the National Security Council to provide readouts of the president’s phone calls with foreign leaders. This one was prepared by the NSC’s Ukraine expert,” deputy press secretary Hogan Gidley said in a statement that implicitly referred to Vindman.

On Sunday, Trump tweeted that Williams, “whoever that is,” should “read BOTH transcripts of the presidential calls & see the just released statement from Ukraine.”

“Then she should meet with the other Never Trumpers, who I don’t know & mostly never even heard of, & work out a better presidential attack!” he wrote around midday.

Several witnesses in the inquiry have already faced aggressive smear campaigns fueled by Trump and his allies. Yovanovitch was removed as ambassador in May following one such effort, and former National Security Council official Fiona Hill told lawmakers last month that she received death threats and other “hateful calls” when she began her job at the White House.

The attacks have been particularly intense for women and officials who were not born in the United States. Yovanovitch was born in Canada, to parents who had fled the Soviet Union and the Nazis. She moved to Connecticut as a child and became a U.S. citizen at 18. Hill, born in the United Kingdom, is a dual citizen. And Vindman fled Ukraine with his Jewish family at age 3 and was raised in Brooklyn.

This month, some conservative commentators used Vindman’s origin story to raise questions about his patriotism. On CNN, former congressman Sean P. Duffy (R-Wis.) said Vindman “has an affinity, I think, for the Ukraine.” On Fox News, host Laura Ingraham described him as “a U.S. national security official who is advising Ukraine while working inside the White House.” John Yoo, a lawyer who served in the Justice Department during the George W. Bush administration, told Ingraham that “some people might call that espionage.”

The charge of dual loyalty — even that Vindman was acting as a “double agent” — quickly circulated in right-wing corners of the Internet, where the president’s supporters coordinate their messaging to project greater influence than they may actually exercise.

In a tweet, Jack Posobiec, a correspondent for the One America News Network, falsely claimed that Vindman was working with his country of origin to thwart Trump’s agenda. He cited the New York Times as his source, though the notion appeared nowhere in the newspaper’s reporting. His false statement quickly gained thousands of retweets, and the same claim was also repeated word-for-word by other accounts.

The attacks resumed as Vindman prepared to testify in public. The Gateway Pundit, a conservative blog, twisted reporting in The Washington Post to falsely accuse Vindman of coordinating with the whistleblower to initiate an “attempted coup of President Trump.”

Impeachment witnesses who were born in the United States have faced fewer and less pointed attacks.

Speaking of Taylor and Kent last week during a rally with supporters in Bossier City, La., Trump mocked them as clueless.

“How about when they asked these two Never Trumpers, ‘Uh, what exactly do you think you impeach him for?’ And they stood there and went like, ‘What?’ ” Trump told the crowd.

Trump’s mid-hearing attack on Yovanovitch appeared to thrill his supporters online. The tweet ricocheted around large pro-Trump Facebook groups where users gathered during the hearing — trading attacks on Democrats and portraying the impeachment inquiry as illegitimate.

Yet despite the applause from Trump’s base, Republican lawmakers either declined to comment, played down the move or expressed polite disagreement with Trump.

“The president’s going to defend himself,” said Rep. Lee Zeldin (R-N.Y.). “Don’t expect this president of the United States to just to sit back and allow this to go on.”

House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) called the tweet “witness intimidation in real time by the president of the United States.” The articles of impeachment for Presidents Bill Clinton and Richard M. Nixon both included charges based on attempts to manipulate witness testimony.

Former independent counsel Kenneth Starr, who headed the investigation that led to Clinton’s 1998 impeachment, argued that it did not rise to the level of witness intimidation but nonetheless showed “extraordinarily poor judgment.”

“I must say that the president was not advised by counsel in deciding to do this tweet,” Starr said on Fox News. “. . . The president frequently says, ‘I follow my instincts.’ Sometimes, we have to control our instincts, so obviously this was, I think, quite injurious.”

Back online, messages from Trump supporters that appeared to inspire the president’s eldest son — “I hired [Trump] to fire people like Yovanovitch” — gained about 12,600 interactions during a roughly 2 1/2-hour period, according to analysis conducted by Marc Owen Jones, a disinformation researcher and assistant professor at Hamad Bin Khalifa University in Doha, Qatar. It continued to spread through much of the afternoon, appearing on other major social media platforms, such as Facebook, as well as on Trump-friendly message boards on 4chan.