Top Pence aide set to testify in impeachment probe

A senior aide to Vice President Pence is poised to testify Thursday in the House's swift-moving impeachment investigation into President TrumpDonald John TrumpWatergate prosecutor says that Sondland testimony was 'tipping point' for Trump In private moment with Trump, Justice Kennedy pushed for Kavanaugh Supreme Court nomination: book Obama: 'Everybody needs to chill out' about differences between 2020 candidates 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 BidenJoe BidenKamala Harris receives new Iowa endorsements after debate performance Watergate prosecutor says that Sondland testimony was 'tipping point' for Trump Overnight Defense — Presented by Boeing — Deal on defense bill proves elusive | Hill, Holmes offer damaging testimony | Trump vows to block Navy from ousting officer from SEALs 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 GiulianiRudy GiulianiOvernight Defense — Presented by Boeing — Deal on defense bill proves elusive | Hill, Holmes offer damaging testimony | Trump vows to block Navy from ousting officer from SEALs Hill, Holmes offer damaging impeachment testimony: Five takeaways Graham requests State Department documents on Bidens, Ukraine 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 BoltonJohn BoltonHill, Holmes offer damaging impeachment testimony: Five takeaways The Hill's 12:30 Report — Presented by Johnson & Johnson — Witness dismisses 'fictional' GOP claims of Ukraine meddling Hill says Bolton suggested Giuliani's actions would 'come back to haunt us' 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 SchiffAdam Bennett SchiffHill, Holmes offer damaging impeachment testimony: Five takeaways Schiff says Trump's actions go 'beyond anything Nixon did' Trump campaign releases 'Bull-Schiff' T-shirts 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.