Pence aide testifies in Trump impeachment inquiry, Bolton a no-show

US congressional committees conducting an impeachment probe 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.

Lawmakers are seeking information from Williams about 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 fired by Trump in September, was called to appear on Thursday, but Bolton's attorney said he would not testify voluntarily and he has not yet been subpoenaed.

A US House of Representatives Intelligence Committee official said on Thursday that Bolton has threatened to take the committee to court if it subpoenas him.

Bolton willing to testify?

The Washington Post, citing people familiar with Bolton's views, said he is willing to testify but 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.

The House investigation is focused on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Biden, a leading Democratic rival at the November 2020 presidential election. Williams was one of a handful of US officials who listened in on the call.

They are trying to determine whether Trump froze $391 million in US security assistance for Ukraine to put pressure on Zelenskiy to conduct the investigation, misusing US foreign policy for his personal gain.

Trump and his backers blast the House inquiry as a witch hunt and accuse Democrats of unfairly targeting him in hope of reversing his surprise election victory in 2016. In a tweet on Wednesday, Trump called the probe a "phoney scam."

Democrats have been releasing transcripts of previous closed-door interviews this week as they prepare the first public impeachment hearings next week.

US diplomats main witnesses

Three US diplomats who expressed alarm about Trump's dealings with Ukraine and have already testified in private will serve as star witnesses.

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.

Setting the stage for the public hearings, lawmakers have been making public transcripts of the closed-door depositions. 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 ousting the president.