Jennifer Williams, a top adviser to Vice President Mike Pence, arrived on Capitol Hill Thursday to testify in the House impeachment probe against the president, as a string of other administration officials have refused to do so.
Lawmakers want to know what Pence knew about efforts by Trump - and those around him - to pressure Ukrainian officials to investigate former Vice President Joe Biden and his son Hunter.
Williams - a career foreign service officer who was on the July 25th call between Trump and Ukraine - is one of the last witnesses to testify behind closed doors before public hearings start next week.
As Williams was arriving on Capitol Hill, President Trump was lashing out on Twitter…
after a report by the Washington Post said he wanted Attorney General William Barr to hold a news conference to say that no laws were broken on Trump's July 25th phone call with the President of Ukraine when he asked for a favor - the subject of the impeachment probe on Capitol Hill.
In a Tweet, Trump wrote: "The degenerate Washington Post MADE UP the story about me asking Bill Barr to hold a news conference. Never happened, and there were no sources!"
According to the Post, Barr did not comply with the request.
Democrats in the House are trying to determine if Trump abused his power, questioning whether Trump withheld $391 million in aid to pressure Ukraine into investigating the Bidens and alleged interference by Ukraine in the 2016 election.
Trump denies any wrongdoing.
Democrats have been releasing transcripts of previous closed-door interviews this week including from top U.S. diplomat to Ukraine, Ambassador Bill Taylor.
Taylor corroborated many of the allegations against Trump, and described an explicit quid pro quo involving U.S. military aid to Ukraine.
Taylor will testify in televised hearings next week.