A career Army officer on Donald Trump’s National Security Council testified Tuesday he was duty-bound to object to the president’s clearly “improper” phone call seeking Ukrainian investigations of U.S. Democrats. Republicans answered him with doubts about his loyalty to the United States.

Arriving on Capitol Hill in military blue with medals across his chest, Lt. Col. Alexander Vindman told impeachment investigators he felt no hesitation in reporting the president’s request of Ukrainian President Volodymyr Zelenskiy.

Vindman, a 20-year military officer who received a Purple Heart for being wounded in the Iraq War, was among the officials who listened in to the July 25 call when Trump asked Ukrainian President Volodymyr Zelenskiy for “a favor” — investigations of Democrat Joe Biden and other issues.

“It was inappropriate, it was improper for the president to request, to demand an investigation into a political opponent,” Vindman told the House Intelligence Committee.

His testimony launched a pivotal week as the House’s historic impeachment investigation reaches further into Trump’s White House.

Democrats say Trump’s pressure on Ukraine to investigate former Vice President Joe Biden while withholding U.S. military aid to Kyiv may be grounds for removing the 45th president. Republicans have argued both that there was no linkage between the two matters and that there would be nothing inappropriate even if there was.

In a remarkable day of back-to-back hearings, Vindman testified alongside Jennifer Williams, an adviser in Vice President Mike Pence’s office. Both said they had concerns as they listened to Trump speak with the newly elected Ukrainian president about political investigations into Biden.

Trump insists Zelenskiy did not feel pressured and has cast the impeachment probe as a partisan affair aimed at pushing him from office. The White House lashed out at the Army officer.

It wasn’t the first time Vindman was alarmed over the administration’s push to have Ukraine investigate Democrats, he testified.

He highlighted a July 10 meeting at the White House when Ambassador Gordon Sondland told visiting Ukraine officials they would need to “deliver” before next steps — a meeting Zelenskiy wanted with Trump.

“Ambassador Sondland referred to investigations into the Bidens and Burisma in 2016,” he testified, referring to the gas company in Ukraine where Joe Biden’s son Hunter served on the board.

On both occasions, Vindman said, he took his concerns about the shifting Ukraine policy to the lead counsel at the NSC, John Eisenberg.

An immigrant who came to the U.S. as a toddler from Ukraine, Vindman opened his testimony by assuring his father that in America he would be “fine for telling the truth.”

Yet Vindman spent long stretches fielding Republican attacks on his loyalty and his career in public service. The Republicans’ lead counsel asked at one point about an offer he got from a Ukrainian official to become the country’s defense minister.

Vindman called it “comical” and said he swiftly reported it up his chain of command.

“I’m an American,” Vindman said. “And I immediately dismissed these offers.”

Later Tuesday, the House committee heard from former NSC official Timothy Morrison and Kurt Volker, the former Ukraine special envoy, who said he hadn’t understood the scope of the investigations Sondland and Rudy Giuliani, the president’s personal attorney, were pursuing for Trump.

Sondland, the U.S. ambassador to the European Union, is to appear Wednesday as the most-anticipated witness yet.

At the White House, Trump said he watched part of the day’s testimony and slammed the ongoing impeachment hearings as a “disgrace.” Over the weekend, he had assailed Williams as part of the “Never Trumpers” who oppose his presidency, though there is no indication she has shown any partisanship.

Vindman was ready to defend his loyalty to the United States. When the top Republican on the committee, Rep. Devin Nunes, addressed him as “Mr. Vindman,” the colonel reminded him to address him by his rank.

He also deflected Republican efforts to get him to divulge everyone he told about the Trump call — thwarting Trump allies’ attempts to identify the anonymous whistleblower who spurred the impeachment probe.

Nunes bore down once Vindman acknowledged one person he talked to was from the intelligence community. The whistleblower is a CIA official, according to people with knowledge of the matter.

Vindman said he does not know who the whistleblower is. He has previously said it is not him.

Trump ally Jim Jordan asked if he ever leaked information. “Never did, never would,” Vindman answered.

Republicans were eager to hear during the afternoon from Morrison, who had supervised Vindman at the NSC. “He had concerns about Vindman’s judgment,” the White House tweeted.

But Morrison, who has since left the administration, told lawmakers he was not there to question his former colleagues’ “character or integrity” and did not intend to out the whistleblower.

Morrison, who was also listening to Trump’s call, worried its disclosure would not play well in polarized Washington, and reported it to the NSC’s top lawyer. He testified about his sinking feeling after Sondland told him Trump wanted Zelenskiy to announce the investigations before releasing the military aid. A colleague warned him of “the Gordon problem,” he said.

Vindman is being provided security by the U.S. Army and local law enforcement, according to a U.S. official. The official said the Army is prepared to take additional steps, if needed, including moving Vindman and his family to a more secure location on a base.

Williams, a career State Department official who has worked for three presidential administrations and counts former Secretary of State Condoleezza Rice a “personal hero,” said the Trump phone call was the first time she had heard anyone specifically seeking investigations from Ukraine.

The reference to Biden and his son Hunter “struck me as political in nature.”

Williams testified the call was unlike about a dozen others she had heard from presidents over her career. When the White House produced a rough transcript later that day, she put it in Vice President Pence’s briefing materials. “I just don’t know if he read it,” she testified earlier in her closed-door House interview.

Pence’s role throughout the impeachment inquiry has been unclear.

The vice president’s national security adviser, Lt. Gen. Keith Kellogg, issued a statement saying he “heard nothing wrong or improper on the call.”

Vindman said Trump’s remarks on the call strayed from the talking points prepared for him. And both witnesses said Zelenskiy had mentioned “Burisma” but it was missing from the rough transcript released by the White House.

At the time of the call, the officials were just beginning to make the link with the stalled military aid — $391 million approved by Congress— that Ukraine was relying on as it confronts neighboring Russia.

Vindman said the uneven power dynamic between the presidents of the East European ally and the U.S. made the demand obvious.

“The culture I come from, the military culture, when a senior asks you to do something … it’s not be taken as a request, it’s to be taken as an order,” he said.