Jennifer Williams, the special adviser to Vice President Pence for Europe and Russia, testifies in closed-door hearing.

US congressional committees conducting an impeachment inquiry of President Donald Trump met on Thursday for the first time with an adviser to Vice President Mike Pence, but former national security adviser John Bolton failed to heed a request to appear.

Jennifer Williams, a career foreign service officer and special adviser to Pence for Europe and Russia, was testifying in a closed-door hearing in front of members of the House of Representatives Foreign Affairs, Intelligence and Oversight committees after receiving a subpoena to compel her testimony.

Politicians are seeking to find out how much Pence knew about efforts by Trump and those around him to pressure Ukraine to investigate former Vice President Joe Biden and his son Hunter, as well as foreign interference in the 2016 US election.

Bolton, who was forced out by Trump in September, was also called to appear on Thursday but did not show, and his lawyer said he would not testify voluntarily.

A US House of Representatives Intelligence Committee official said that Bolton has threatened to take the committee to court if it subpoenas him. A congressional source said the inquiry is unlikely to go down that route.

Bolton’s office and his lawyer did not respond to a request for comment.

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The Washington Post, citing people familiar with Bolton’s views, said although he is willing, he wants to see how a court battle between Congress and the White House over the constitutionality of the subpoenas shakes out first. The battle is likely to go to the Supreme Court and could spill into next year.

Members of the committees conducting the inquiry have said they want to see if Bolton will corroborate previous witnesses’ testimony that he was alarmed at Trump asking a foreign government to get involved in domestic politics.

‘Quid pro quo’

The House’s investigation, which started with a complaint from a whistle-blower, is focused on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskyy to investigate Biden, a leading Democratic rival in the November 2020 presidential election. Williams was one of a handful of US officials who listened in on the call.

The House panels are trying to determine whether Trump froze $391m in US security assistance for Ukraine to put pressure on Zelenskyy to conduct the investigation, misusing US foreign policy for his personal gain.

President Donald Trump meets with Ukrainian President Volodymyr Zelenskyy at the InterContinental Barclay New York hotel during the United Nations General Assembly [File: Evan Vucci/AP Photo]

Trump’s defenders say there is no evidence of him and the Ukrainian president engaging in a “quid pro quo” (Latin for “a favour for a favour”) because the aid to Ukraine was released and Zelenskyy never explicitly promised anything.

A quid pro quo is not necessary to prove high crimes or misdemeanours, which is the standard the Constitution requires for the impeachment of a president.

Republicans have also accused Democratic House leaders of holding the inquiry in secret because they are closed-door, although Republican committee members have been present throughout.

Republicans have been invited to submit requests for witnesses in the first public hearings that start next week and are expected to ask to hear from the whistle-blower.

Public hearings

Adam Schiff, the Democratic chairman of the House intelligence panel, said the requests are due by Saturday.

In a letter sent on Wednesday to Devin Nunes, the top Republican on the House Intelligence Committee and a strong Trump supporter, the whistle-blower’s lawyers reiterated that their client was willing to answer written questions.

Democrats have been releasing transcripts of the closed-door interviews as they prepare for the public hearings.

Three US diplomats who have already testified in private about their alarm about Trump’s dealings with Ukraine will serve as star witnesses next week.

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William Taylor, the top US diplomat in Ukraine, and George Kent, another career diplomat with experience in the country, will testify on November 13. Marie Yovanovitch, who was abruptly pulled from her post as US ambassador to Ukraine in May, will testify on November 15.

Sources familiar with the investigation said Kent’s transcript would be made public on Thursday.

If the Democratic-controlled House votes to impeach Trump, the Republican-controlled Senate would then hold a trial on whether to remove him from office.

Senate Republicans have so far shown little appetite for removing the president.