A top aide to Vice President Pence is expected to testify Thursday in the ongoing House impeachment inquiry of President Trump.

Jennifer Williams is a longtime foreign service officer who was assigned to be the vice president's special adviser for Europe and Russia in the spring. She would be the first person from the vice president's office to testify in the probe of whether the president withheld military aid from Ukraine while seeking a political favor.

She will be the third person who was listening in on the July 25 call between Trump and Ukrainian President Volodymyr Zelenskiy to testify, a White House aide confirmed to NPR's Mara Liasson.

Williams is scheduled to testify on the same day as former national security adviser John Bolton. But investigators have only scheduled 30 minutes for Bolton's deposition, an indication they do not expect him to show up.

House investigators are likely to ask Williams about the July call as well as a September trip to Poland where Pence met with Zelenskiy.

Williams referred questions to her attorney, but said she didn't expect to release an opening statement. Her lawyer, Justin Shur, said her testimony is likely to correspond with prior witnesses' testimony.

"Jennifer is a longtime dedicated State Department employee," Shur told NPR. "If required to appear, she will answer the Committees' questions. We expect her testimony will largely reflect what is already in the public record."

Those who know Williams describe her as a smart and ideal diplomat.

Brett Bruen, the former director of global engagement at the White House under President Barack Obama, trained with Williams when they both worked at the State Department.

He said Williams will be honest and won't hold back. He expects Williams will ultimately put the interests of the country and national security institutions before the interests of any individual, including the president or vice president.

"She's your ideal example of an diplomat," Bruen said. "Cool under pressure, confident, super smart and deeply committed to the mission."

Williams traveled with Pence to Warsaw for the Sept. 1 meeting where they discussed the $400 million in stalled security aid.

The security aid was at the top of Zelenskiy's mind when he met with Pence, according to testimony of Ambassador William Taylor, the top U.S. diplomat in Ukraine.

Zelenskiy "opened the meeting" by asking Pence about security cooperation, Taylor testified.

"The Vice President did not respond substantively but said he would talk to President Trump that night," Taylor told investigators, according to the released transcript. "The Vice President did say that President Trump wanted the Europeans to do more to support Ukraine and that he wanted Ukrainians to do more to fight corruption."

Pence told reporters the next day that he didn't discuss Biden with Zelenskiy. But he said they did discuss "corruption" and "the upcoming decision the President will make on the latest tranche of financial support."

"But as President Trump had me make clear, we have great concerns about issues of corruption," Pence said.

Pence has struggled to answer directly whether he had any knowledge of an alleged quid pro quo with Ukraine to investigate the son of former Vice President Joe Biden.

When asked whether he thought Taylor and other U.S. officials were lying about a pressure campaign, Pence told CBS's Face the Nation that the public should read the transcript of the president's call with his Ukrainian counterpart.

"Well, I can only tell you what I know," Pence said in the Oct. 27 interview. "And what I know is that the transcript of the president's call with President Zelenskiy shows that there was no quid pro quo. He did nothing wrong."

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