WASHINGTON (AP) — 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.