Two witnesses testifying at Tuesday morning's impeachment inquiry hearing flagged concerns about President Donald Trump's July 25 call with Ukraine's president as Democrats and Republicans sparred over the identity of a whistleblower, the use of the word "bribery" and accusations of questioning a White House official's loyalty to the U.S.

Army Lt. Col. Alexander Vindman, the top Ukraine expert on the Nation Security Council, and Jennifer Williams, a foreign service officer and aide to Vice President Mike Pence, are the latest witnesses to testify before the House Intelligence Committee in Democrats' impeachment inquiry looking into whether Trump abused the powers of his office by pushing Ukraine on a July 25 call to open political investigations and whether military aid was conditioned on those probes.

Vindman called the July 25 call between Trump and Ukrainian President Volodymyr Zelenskiy "improper" and testified that he had a "duty" to report the call to White House counsel John Eisenberg.

"Without hesitation, I knew I had to report my concerns to the White House counsel. As I said in my statement, it was improper for the president to demand an investigation into a political opponent, especially from a foreign power where there's at best dubious belief that this would be a completely impartial investigation," Vindman said.

"It was also clear that if Ukraine pursued an investigation into the 2016 election, the Bidens and Burisma, it would be interpreted as a partisan play," Vindman added, referring to the administration and Trump personal lawyer Rudy Giuliani's push for probes into a Ukrainian gas company where former Vice President Joe Biden's son held a seat on the board and into uncorroborated allegations that Ukraine, not Russia, interfered in the 2016 election.

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Vindman also testified that Trump didn't stick to his drafted talking points about addressing corruption on his call with Zelenskiy. But the NSC official also said that he didn't think there was anything "nefarious" about putting the record of the call on a highly classified server. Vindman also said that leaving Burisma out of the White House memo of the call was "not a significant omission" and that he believed the transcript was "substantively accurate."

Meanwhile, Williams testified that she thought the July 25 call was "unusual" and "different" because Trump discussed domestic politics on a call with a foreign leader.

"Reflecting on what I was thinking in that moment, it was the first time I heard internally the president mention particular investigations that I had only heard from Giuliani's press interviews," Williams said, adding that it "wasn't clear if there was a direct connection or linkage on security assistance, but it was noteworthy in that regard. I didn't have enough information to draw further conclusions."

"I thought that the references to specific individuals and investigations struck me as political in nature, given that the former vice president is a political opponent of the president," she added.

Intelligence Committee Ranking Member Devin Nunes opened his line of inquiry by attacking the media and accusing last week's witnesses as "three diplomats who dislike President Trump's Ukraine policy." The beginning of Republican questioning focused on Burisma and the Bidens. Nunes also asked both witnesses about whether they discussed the July 25 call with the press. Vindmand and Williams said they didn't speak with the press.

During the extended questioning, Republican counsel Steve Castor questioned Vindman over an offer he received to be Ukraine's minister of defense. Vindman said he reported the offer and never entertained it because "I am an American."

"The whole notion is rather comical that I was being asked to consider whether I would want to be the minister of defense. I didn't leave the door open at all," Vindman said.

Democrats later called out Castor's questioning on the offer with Rep. Jim Himes of Connecticut, saying that it was "cloaked in a Brooks Brothers suit and parliamentary language" but argued it was really about questioning Vindman's loyalty. Some Democrats have accused Republicans of insinuating that Vindman has dual loyalties because he immigrated to the U.S. from the Soviet Union when he was a child.

Throughout the hearing, Democrats and Republicans had some tense exchanges, especially when it came to the intelligence community whistleblower, who lodged a complaint about the July 25 call which sparked Democrats' impeachment inquiry into the president.

While Vindman vigorously pushed back that he never discussed the call with the press, he noted that he discussed the July 25 call with two people with an "appropriate need to know." He mentioned George Kent, a State Department official who testified before the impeachment committee last week, and an individual in the intelligence community.

House Intelligence Committee Adam Schiff of California interrupted the ranking member's questioning and said the committee needs to "protect" the identity of the whistleblower. Vindman testified that he doesn't know the identity of the whistleblower but that he's been instructed not to discuss his communication with anyone in the intelligence community.

"These proceedings will not be used to out the whistleblower," Schiff said.

Republicans also sought to blow a hole in Democrats' argument that Trump engaged in "bribery" by conditioning nearly $400 million in military aid on Ukraine opening investigations into the Bidens and the 2016 election – recently pivoting from language that it was a quid pro quo.

Rep. John Ratcliffe of Texas, who printed out thousands of pages of witness depositions, said that the word "bribery" only came up once in all those transcripts.

Schiff later responded that "bribery does involve quid pro quo," adding that it isn't the job of the witnesses appearing before his committee to determine if a crime was committed.