A senior aide to Vice President Pence is poised to testify Thursday in the House's swift-moving impeachment investigation into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's handling of foreign policy in Ukraine.

Jennifer Williams, a career foreign service officer and top Pence adviser, will be a rare witness to comply with Democrats' request to appear amid a week in which a host of senior officials have refused to testify in the probe.

An official working on the impeachment inquiry says Williams was subpoenaed for testimony Thursday morning after the White House sought to direct her not to appear for her deposition, and in order to prevent the administration from seeking to limit what she will say to House investigators.

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"As required of her, Ms. Williams is complying with the subpoena and answering questions from both Democratic and Republican Members and staff," the official said.

Williams was among the officials who was on the July 25 phone call between Trump and Volodymyr Zelensky, during which Trump asked the Ukrainian president for a favor: he wanted Kiev to open anti-corruption investigations into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son Hunter Biden, who was employed by a Ukrainian energy giant in the Obama era.

That call prompted the complaint from an anonymous government whistleblower charging that Trump had sought to enlist a foreign leader for help in targeting a domestic political rival — and boosting his reelection chances next year. The complaint — alleging that Trump had threatened to withhold U.S. military aid to Ukraine to exert additional pressure on Zelensky — triggered the launch of the Democrats' impeachment investigation in late September.

A number of top State Department officials have already testified to what they viewed as a clear quid pro quo.

Williams is sure to be pressed by lawmakers on the extent to which Pence played a role in the pressure campaign, led by Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, Trump's personal lawyer, who had precipitated the removal of Marie Yovanovitch, U.S ambassador to Ukraine, in May.

Trump and his GOP allies on Capitol Hill have insisted the president's dealings with Zelensky were not politically motivated, but merely sought to ensure that taxpayer dollars were not lost to corruption in a country known for it.

"It’s all a hoax. It’s a scam," Trump said Wednesday night during a rally in Louisiana.

Williams's appearance follows on the heels of another deposition featuring David Hale, the third-ranking official in the State Department, who bucked the White House's attempted blockade on cooperation with the impeachment probe by testifying for more than five hours on Wednesday.

Democrats have also requested the appearance Thursday of John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, Trump's former national security adviser who was pushed out in September. Bolton's lawyer has said his client will not testify voluntarily, but left open the possibility that he would appear under subpoena. If was unclear Thursday morning if Democrats have issued such a subpoena — or if Bolton would comply even if they did.

The interviews are expected to be among the last conducted behind closed doors, as Democrats have announced their intention to open up the impeachment investigation next week with a series of public hearings featuring some of the same figures who have already testified privately in the probe.

House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) said that William Taylor, the top U.S. diplomat to Ukraine, and George Kent, a top State Department official, will testify next Wednesday, while Yovanovitch is scheduled to appear next Friday.

— This report was updated at 10:10 a.m.