The House impeachment hearing Tuesday is set to feature one of the Democrats’ star witnesses: Lt. Col. Alexander Vindman, a Purple Heart recipient who is expected to testify about his alarm at President Trump’s request that Ukraine investigate his political opponents.

But Republicans are also seizing on Vindman’s testimony as an opportunity, signaling that they plan to try to discredit one of the key witnesses in the inquiry by questioning his motives and his loyalty to the president.

The high-stakes appearance could provide some of the most contentious moments in the public hearings and set the tone as the inquiry heads into a busy week of testimony.

The spotlight will intensify the pressure on Vindman, who continues to serve in the White House as the top Ukraine expert on the National Security Council. He previously told House lawmakers that he believed the president crossed a “disturbing” line when he asked the Ukrainian president for a “favor” during a July 25 call — that Ukraine investigate his Democratic rival Joe Biden.

Vindman, who was among those listening to the conversation, was so concerned that within an hour, he warned White House lawyers that the president’s actions were improper.

Since he emerged as a witness last month, Trump and his allies have denounced Vindman. The president called him a “Never Trumper witness,” and on Fox News, Laura Ingraham described Vindman as “a U.S. national security official . . . working inside the White House, apparently against the president’s interests.”

The attacks picked up again on the eve of his testimony. Sen. Ron Johnson (R-Wis.) suggested in a letter released Monday that Vind­man fits the profile of “a significant number of bureaucrats and staff members within the executive branch [who] have never accepted President Trump as legitimate and . . . react by leaking to the press and participating in the ongoing effort to sabotage his policies and, if possible, remove him from office.”

Vindman’s lawyer, Michael Volkov, called Johnson’s assertion “such a baseless accusation, so ridiculous on its face, that it doesn’t even warrant a response.”

“Lt. Col. Vindman is a patriotic veteran, awarded the Purple Heart, who has selflessly served this country for over 20 years,” Volkov added.

The attacks have upset Vindman and his twin brother, who both enlisted in the U.S. Army and launched careers in government, with both assigned to the White House National Security Council, according to a person familiar with Vindman’s thinking who spoke on the condition of anonymity because of the ongoing inquiry.

He plans on emphasizing his service to the country, as he did during his closed-door testimony last month, said the person, who stressed that he has no animus toward the president and describes himself as nonpartisan.

“I am a patriot, and it is my sacred duty and honor to advance and defend our country, irrespective of party or politics,” Vindman told lawmakers in October. “I have dedicated my entire professional life to the United States of America.”

That theme is expected to be echoed by Democrats, who will seek to portray him as a lifelong public servant who took the risk of alerting his superiors to a presidential action that he believes ran counter to U.S. national security interests and possibly the law.

Republicans plan to cast Vindman as a low-level aide who operated outside proper channels — and someone who they suggest may have been agitating against Trump and leaking information.

On Monday evening, the ranking Republican on the Judiciary Committee, Rep. Douglas A. Collins (R-Ga.), released a letter expressing “concern regarding the credibility and judgment” of Vindman.

“According to the transcript, officials on the National Security Council were concerned about Colonel Vindman’s judgment and about whether he may have leaked information,” Collins wrote in a letter to Judiciary Committee Chairman Jerrold Nadler (D-N.Y.).

GOP lawmakers face a delicate challenge in discounting the testimony of the 44-year-old Iraq War veteran, who will be wearing his dress blue uniform and a chest full of medals Tuesday.

In a sign of the sensitivity of the matter, the Republican leadership was personally involved in developing the strategy for questioning Vindman, one person familiar with the discussions said. As of late Monday afternoon, the plan for handling Vind­man had not yet been finalized.

One subject GOP lawmakers are expected to probe is with whom Vindman discussed the July 25 call.

When he testified behind closed doors, Republican lawmakers raised questions about whether Vindman was a source for the whistleblower, a CIA analyst whose complaint set in motion the impeachment inquiry.

Vindman has said he spoke to several people in the aftermath of the July 25 call about his concerns, including State Department official George Kent. He is expected to testify, as he has before, that he does not know the identity of the whistleblower.

GOP lawmakers are also expected to spend some of their time Tuesday highlighting how differently Vindman interpreted and reacted to the July 25 call, compared with two other witnesses who were also listening in and are scheduled to testify Tuesday. Neither Jennifer Williams, an aide to Vice President Pence, nor Tim Morrison, the senior director for Russian affairs at the National Security Council, lodged a complaint with their superiors or warned White House lawyers, as Vindman did.

To prepare for Tuesday’s hearing, Vindman reviewed the testimony of Morrison, his former immediate supervisor, who was sometimes at odds with Vindman before he departed the White House last month.

Morrison was critical of Vindman, telling lawmakers that Vind­man had gone directly to a White House lawyer to complain rather than through the chain of command. However, White House attorney John Eisenberg had previously told Vindman to report any concerns to him.

Morrison also questioned Vindman’s “judgment,’’ including a willingness to speak freely about sensitive matters.

In contrast, Vindman received a stellar performance rating from his previous boss, Fiona Hill, in his last annual job evaluation, according to a copy reviewed by The Washington Post.

“Superb performance from an outstanding director,” Hill wrote in Vindman’s performance review dated July 2019. “Alex’s unparalleled expertise, on all aspects of his portfolio, make him an indispensable member of the National Security Staff.”’

She described him as “a consummate teammate and prized adviser” who “stepped into the most complex, fast-paced unforgiving environment in the U.S. government and excelled.”

Morrison and Hill both testify this week, and both could be quizzed further on their views of Vindman. Hill is expected to say that she thought very highly of Vindman and his abilities and to praise his skillful execution of complex tasks, according to a person familiar with her account; she has acknowledged to allies that as a military officer, Vindman had a strong sense of duty and accomplishing his mission but didn’t need to be a participant in complex political disputes.

Greg Miller and Greg Jaffe contributed to this report.