Editor's Note: (This story first published November 5 ahead of Jennifer Williams' closed-door deposition. It has been updated with additional developments after her initial testimony.)
Washington(CNN) A senior adviser to Vice President Mike Pence who complied with a request to testify earlier this month in front of the committee leading the impeachment inquiry was disparaged by President Donald Trump on Sunday, two days before she is set to testify publicly before the panel.
Jennifer Williams, who was the first person on Pence's national security team to appear, and knowledge of how much the vice president knew about the efforts by Trump and those around him to push Ukraine to launch investigations into Joe Biden and his son, as well as 2016 election interference.
During a closed-door deposition earlier this month, Williams told lawmakers she had been in the White House Situation Room listening to Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky in real time and reviewed a transcript of Trump's April call with Zelensky prior to Pence's own April call with the then-Ukrainian President-elect. Williams also listened to a second call between Pence and Zelensky, which took place on September 18 after a US hold on security assistance to the country had been lifted. She testified that Trump's request for specific investigations struck her as "unusual and inappropriate" and "shed some light on possible other motivations" for his decision to freeze security aid to Ukraine.
Tweeting a day after Williams' earlier, closed-door testimony was released, Trump resurfaced an unfounded accusation he has raised against other officials who have testified in the probe, characterizing Williams as a Never Trumper and associating her with other "Never Trumpers." The President also urged Williams to "read BOTH transcripts of the presidential calls."
Pence's office on Sunday declined to defend Williams after Trump's Twitter attack, with Pence's press secretary Katie Waldman simply telling CNN: "Jennifer is a State Department employee."
Williams is scheduled to appear before the House Intelligence Committee on Tuesday to testify publicly as part of the impeachment probe.
Williams, a longtime State Department staffer, is detailed to Pence's office as special adviser on European and Russian affairs and was one of two Pence aides on the call. The other was Gen. Keith Kellogg, the vice president's national security adviser, who has not yet been called to testify.
Pence did not listen in, but a transcript of the call was put into his daily briefing binder, an administration source says.
More than a month after the call, Williams traveled with Pence to Warsaw, Poland, when he stood in for Trump and met with Zelensky on September 1. After that meeting, Pence said they had discussed corruption in Ukraine and he evaded a question about whether the holdup of almost $400 million in security aid for Ukraine was tied to efforts by Rudy Giuliani, the President's personal attorney, to dig up dirt on Joe and Hunter Biden. There is no evidence of wrongdoing by either Biden in Ukraine.
Since then, Pence has insisted that Trump did nothing wrong but has not clarified how much he knew about efforts to pressure Ukraine and the parallel Ukraine policy Giuliani and others were leading outside the normal diplomatic and official channels.
Current and former colleagues have heaped praise on Williams, with one White House official saying: "She is the most professional person in this building."
The official noted that Williams had recently worked with Pence on negotiating the ceasefire between Turkey and Syrian Kurdish forces. Until Williams joined Pence's staff, she was a spokesperson for several years at the US embassy in London, according to her LinkedIn profile. She also held posts at the embassies in Beirut, Lebanon and Kingston, Jamaica.
"Jen is the type of person who I know to be dedicated to the institution above anything else," said Brett Bruen, the former Director of Global Engagement at the White House under President Barack Obama.
She comes from the cadre of diplomats "who are not going to be easily intimidated or hold back information," Bruen added. "She is someone who is pretty tough and has had tough assignments that have steeled her to the kind of pressure and the intimidation tactics that have been used against others."
CLARIFICATION: This story has been updated to more precisely reflect Williams' role.