President Trump on Thursday excoriated an unidentified whistleblower and the White House aides who informed their complaint as “almost a spy” and likened their work to treason — part of a scorched-earth strategy he is directing for the Republican Party at the outset of an impeachment showdown.

Trump has acted impulsively and indignantly as he wages an all-out political war to defend himself from allegations that he abused his power to solicit foreign interference in his 2020 reelection bid.

And in a testament to how completely he controls the Republican Party, many GOP officeholders and conservative media figures have followed Trump’s cues by joining his attempts either to attack the anonymous whistleblower, discredit the explosive accounts in their complaint, or malign the media for covering it.

The coming weeks will test whether Trump’s familiar blame-the-accuser-and-counterattack playbook — while floating dark conspiracies to divert attention — will be successful in the face of mounting evidence that he asked Ukraine to dig up dirt on potential 2020 Democratic opponent Joe Biden.

Acting director of national intelligence Joseph Maguire testified Sept. 26 before the House Intelligence Committee after a whistleblower complaint about Trump. (Monica Akhtar/The Washington Post)

Trump’s own appointed intelligence chief testified Thursday to the whistleblower’s credibility, and some of the allegations in the complaint were confirmed by the rough transcript of Trump’s July 25 call with Ukrainian President Volodymyr Zelensky that the White House released Wednesday.

Long indifferent to the norms that restrained his predecessors, Trump shattered another one Thursday by alluding to violence and possible death for the whistleblower’s sources in an apparent warning to witnesses inside the government who might be asked to testify against him.

Addressing employees of the U.S. Mission to the United Nations during a closed-door meeting in New York, Trump said, “I want to know who’s the person, who’s the person who gave the whistleblower the information? Because that’s close to a spy.”

“You know what we used to do in the old days when we were smart? Right? The spies and treason, we used to handle it a little differently than we do now,” Trump said, according to an audio recording published by the Los Angeles Times. An attendee confirmed the comments to The Washington Post.

Trump has been setting the tone for his party, driving news coverage as ever with a blizzard of provocative comments and dashing any hopes Republican leaders may have had for a more measured impeachment strategy.

“I took a long walk around the [House] floor tonight, talking to dozens of members, and I haven’t seen this level of intensity of support for the president since the 2016 campaign,” Rep. Mark Meadows (R-N.C.), a Trump loyalist, said late Thursday. “We’re with him and ready to go.”

Trump’s advisers said they envision a “split screen” strategy in the coming weeks. The president is considering stepping up his fall schedule of campaign rallies at arenas across the country to galvanize his supporters outside of Washington and portray House Democrats as liberal insiders who are focused on impeachment instead of governing.

Current and former White House officials summed up the mood inside the president’s inner circle as battle-scarred but determined, taking some solace that the president has survived past battles — from the special counsel Russia investigation to the federal probe of hush-money payments — that had been considered political death knells.

“This has been a presidency defined by fights and battles and fending off attacks and attacking back, so in a sense this is the next phase,” said a former senior White House official, who spoke on the condition of anonymity to offer a candid assessment. “There’s been a bunker mentality. That has come to be the normal state of affairs.”

Trump has informally discussed with aides whether to establish a war room inside or outside his understaffed White House to manage the impeachment inquiry, as President Bill Clinton had in the late 1990s. Corey Lewandowski, Trump’s controversial former campaign manager, has been mentioned as a possible hire, but no decisions have been made.

“I have had no conversation with anyone at the White House regarding this,” Lewandowski said. “If the president asks me to push back on the fake impeachment narrative, I will do that in any way I can.”

Trump is single-handedly shaping his party’s emerging impeachment response strategy, reflecting the calculation among Republicans on Capitol Hill that his political base is required to carry the party through the 2020 elections, even if some lawmakers privately disapprove of his conduct.

“He remains the leader of the party, period, and the party is even more with him than ever,” said Rep. Jim Jordan (R-Ohio).

During his questioning of acting director of National Intelligence Joseph Maguire at Thursday’s House Intelligence Committee hearing, ranking Republican Devin Nunes (Calif.) badgered Democrats over what he termed an “information warfare operation against the president.”

“Once again, the Democrats, their media mouthpieces and a cabal of leakers are ginning up a fake story with no regard to the monumental damage they’re causing to our public institutions and to trust in government — and without acknowledging all the false stories they propagated in the past,” Nunes said.

Yet the hearing exposed some cracks in Trump’s support. Rep. Michael R. Turner, a rank-and-file Republican from Ohio, said, “I want to say to the president: This is not okay. That conversation is not okay. And I think it’s disappointing to the American public when they read the transcript.”

Still, most GOP lawmakers unwilling to parrot the president’s lines Thursday simply strove to stay silent. In the halls of the Capitol, “haven’t read the report” was a common refrain. That was the answer to reporters’ questions about the whistleblower complaint from at least five Republican senators: Lamar Alexander (Tenn.), Joni Ernst (Iowa), Rob Portman (Ohio), Tim Scott (S.C.) and Thom Tillis (N.C.).

“If you do want to talk about ethanol, I am happy to talk about ethanol,” Ernst offered.

When Sen. Ted Cruz was asked whether he was ready to be on the “front lines” to defend Trump, the Texas Republican deflected the question and said flatly that he was working for “the people of Texas.”

After making critical comments of Trump’s conduct with Zelensky earlier in the week, Sen. Mitt Romney (R-Utah) told reporters Thursday, “Nothing to add.”

After a splashy warning Wednesday about Republicans “rushing to circle the wagons” around Trump, Sen. Ben Sasse (R-Neb.) rendered himself mute Thursday. He navigated the Senate halls with a phone pressed to his ear, always talking and making himself unable to field any questions.

If the Democrat-controlled House votes to impeach Trump, Senate Republicans will have to decide whether to form a bulwark protecting Trump or to convict him, which would require a two-thirds majority vote in the upper chamber.

“Republicans will be judged by history on whether they were judicial and operating on behalf of the United States or just rubber-stamping Trump’s behavior,” historian Douglas Brinkley said. “It puts Republicans in the Senate in a moral conundrum.”

Trump’s intraparty foes are actively working to take advantage of this juncture. GOP strategist Stuart Stevens, who supports former Massachusetts governor Bill Weld’s long-shot primary challenge to Trump, said he and other Trump critics want to “elevate this issue” among Republicans, despite appearing futile at times.

“Do you want to drop down into the sewer to defend Donald Trump?” Stevens said of fellow Republicans. “Nobody gets out of that sewer smelling the same as when they dropped into it. He’d do nothing for these people, cares nothing for them. It’s all transactional. Why are they putting their political lives on the line for someone they wouldn’t have had dinner with three or four years ago?”

White House advisers said they have high confidence that Senate Republicans will have the president’s back. They cited the political bond Trump has built with Senate Majority Leader Mitch McConnell (R-Ky.) as they have installed scores of conservatives in the federal judiciary.

When asked about Romney’s latest expression of alarm, one White House official mocked him as the “junior senator from Utah.” But the group of lawmakers West Wing aides see as most susceptible to turning on Trump are retiring Republicans in the House and Senate, because they may not fear the president’s wrath as much as their colleagues standing for reelection.

A key element of Trump’s strategy has been to try to shine an uncomfortable light on Biden by making unsubstantiated claims of corruption about the former vice president and his son Hunter. Nearly every set of public remarks Trump has made this week has been heavy on Biden references and insinuations.

Former House speaker Newt Gingrich, a Trump ally, said it was smart for Republicans to “focus on Biden and make the Democrats look ridiculous.”

A legislative pursuit is also in the works: Sen. Ron Johnson (R-Wis.), who chairs the Homeland Security and Governmental Affairs Committee, told reporters this week that he has “some real legitimate questions” about the Bidens’ connections to Ukraine and is exploring a formal investigation.

“Listen, I want to get to the truth,” Johnson said. “I’m not interested in show trials.”

Paul Kane contributed to this report.