By Dustin Volz, Andrew Restuccia, Siobhan Hughes, Wall Street Journal–

Testimony of a White House national security official turned combative Tuesday as Democrats accused Republicans of trying to unmask the whistleblower who sparked the impeachment inquiry of President Trump, while the GOP lawmakers argued theirs was a legitimate line of questioning.

The appearance by Army Lt. Col. Alexander Vindman, a White House Ukraine expert who testified that he expressed alarmed at a U.S. pressure campaign on Ukraine’s president over the summer, spurred Mr. Trump and his allies to attack his background and credentials, which drew some pushback from Republicans.

Meanwhile, House Democrats released a resolution Tuesday laying out how public hearings in their impeachment investigation will be conducted, ahead of an expected vote later this week.

Democrats after Col. Vindman’s hearing called the GOP attempt to ferret out details on the whistleblower potentially dangerous. While the identity of the person remains publicly unknown, the legal team representing him has received multiple death threats that have led to at least one law-enforcement investigation, according to people close to the legal team, as well as other abusive communications.

None of the threats thus far have appeared to be actual credible threats of violence, the people said. The Federal Bureau of Investigation declined to comment.

Mr. Trump has repeatedly lashed out at that whistleblower, known to be a Central Intelligence Agency officer, while pushing for his identity to be revealed. “Where’s the Whistleblower?” Mr. Trump asked on Twitter Tuesday. Since the whistleblower’s account was released more than a month ago, several Trump diplomats and other witnesses have testified, backing up many of the allegations about the pressure campaign on Ukraine while aid to the country was being held up.

Col. Vindman’s hearing occurred behind closed doors, and he said in his prepared testimony that he isn’t the whistleblower and doesn’t know who it is, but Democrats said Republican questioning was aimed at the person’s identity.

“Most of their hour seemed to have been spent trying to backdoor him into narrowing down, for them, who the whistleblower is,” Rep. Debbie Wasserman Schultz (D., Fla.) said.

In a July call, Mr. Trump pressed Ukrainian President Volodymyr Zelensky to open investigations into matters related to the 2016 election and Democratic presidential candidate Joe Biden. Those requests, and the temporary hold on aid to Ukraine, are at the center of House Democrats’ impeachment probe. Mr. Trump has defended the call as appropriate.

Col. Vindman, who listened in on the call, testified that he was concerned because he didn’t think it was appropriate to ask another country to investigate a U.S. citizen. He said he raised concerns to colleagues inside the White House about not just the July 25 call but also a meeting two weeks earlier that he attended between White House and Ukrainian officials. After both, Col. Vindman said he reported his concerns to the lead counsel for the National Security Council.

Republicans complained that Intelligence Committee Chairman Adam Schiff (D., Calif.) had blocked their questioning over whom Col. Vindman had spoken to about his concerns.

“He’s under subpoena—he’s supposed to answer the questions we have during our hour, and Chairman Schiff instructed him not to answer those questions,” said Rep. Jim Jordan (R., Ohio).

The rough transcript of the call released by the White House contained missing words and phrases, Col. Vindman told investigators. He testified that he tried to restore those words but was not successful, according to people familiar with the testimony. The people added it was not clear how significant or intentional the omissions were, and declined to discuss their substance.

The New York Times first reported the testimony regarding the omissions.

But the substance of Col. Vindman’s testimony was at times overshadowed as Mr. Trump and his defenders repeated a pattern of attacking those who come forward in the inquiry that has become a threat to his presidency.

In separate interviews on CNN and Fox News respectively, Sean Duffy, a former Republican congressman, and John Yoo, a former George W. Bush administration official, appeared to suggest that Col. Vindman had divided loyalties because he was born in Ukraine. The Iraq war veteran, who was awarded a Purple Heart, migrated to the U.S. with his family when he was young.

Both Messrs. Duffy and Yoo subsequently sought to walk back their comments.

Republican lawmakers defended Col. Vindman, calling him a patriot. House Republican Conference Chair Liz Cheney said the attacks on Col. Vindman were “shameful.”

“I’m not going to question the patriotism of any of the people who are coming forward,” Senate Majority Leader Mitch McConnell said.

Mr. Trump and his allies launched similar attacks on Bill Taylor, the acting U.S. ambassador to Ukraine, after he told Congress this month that the president made nearly $400 million in aid contingent on Mr. Zelensky’s investigating Mr. Biden and interference in the 2016 U.S. election.

“He’s a ‘Never Trumper,’ and his lawyers are ‘Never Trumpers,’ ” Mr. Trump told reporters last week, a term that typically describes Republicans who didn’t back the president when he ran in 2016. Before rejoining the government at the request of Secretary of State Mike Pompeo, Mr. Taylor had spent nearly five decades serving both Republican and Democratic presidents.

Despite the criticism from the president, a person close to Mr. Taylor said the longtime diplomat had received overwhelming support for his testimony, including emails from the public and members of the military thanking him.

Other witnesses have faced public blowback for getting caught in the Trump administration’s approach to Ukraine. Earlier this month, Rep. Earl Blumenauer (D., Ore.) called for a boycott of the hotel chain founded by Gordon Sondland, the U.S. ambassador to the European Union, whose testimony in the probe has been contradicted by three different witnesses.

The House Democratic resolution released Tuesday sets out parameters for their inquiry going forward. The resolution sets equal time for questioning for both the majority and minority party, of up to 45 minutes per side, and allows Republicans to request witnesses, subject to approval by the Democrats. It also gives the president or his counsel an opportunity to participate in hearings held by Judiciary Committee, including the ability to raise an objection to testimony and cross-examine witnesses.

The resolution authorizes the Intelligence Committee to make transcripts of its recent depositions public, with certain redactions for classified or other sensitive information. A vote on the resolution is expected on Thursday, according to Democratic aides.

The White House said in a statement Tuesday that the resolution showed “Democrats refuse to provide basic due process rights to the Administration.”

A federal judge late last week determined that the House inquiries have legal standing as an impeachment investigation.

Robert Blair, a senior adviser to acting White House chief of staff Mick Mulvaney, is expected to testify in private on Friday, an official working on the impeachment inquiry said. Mr. Blair was among the officials who listened in on the July 25 Trump-Zelensky call.

Mr. Mulvaney in a briefing earlier this month linked the hold on aid to investigations the president wanted Ukraine to pursue, before later walking that back. The White House didn’t respond to a request for comment on Mr. Blair’s testimony.

National-security lawyers said they often warn whistleblowers or witnesses in high-profile investigations about the scrutiny and intimidation they will likely face if they try to raise allegations of wrongdoing.

“We always tell potential whistleblowers that choosing to blow the whistle is a family decision, because it will upend their professional lives,” said Irvin McCullough, a national-security analyst at the whistleblower protection group Government Accountability Project. “They paint a target on their back when they come forward.”

Write to Dustin Volz at dustin.volz@wsj.com, Andrew Restuccia at Andrew.Restuccia@wsj.com and Siobhan Hughes at siobhan.hughes@wsj.com