Donald Trump's personal lawyer Rudy Giuliani on Tuesday brushed off a subpoena from the congressional impeachment investigation, ratcheting up tensions as Democrats build evidence of presidential abuse of office.
Giuliani declared the inquiry "illegitimate" and "unconstitutional," risking being declared in contempt of the Congress as he refused to turn over documents demanded by three Democrat-led committees.
US media said the Office of Management and Budget had also decided not to meet Tuesday's deadline for turning over documents which Democrats believe will help show Trump held back US aid to Ukraine as he demanded Kiev open a corruption probe into his political rival Joe Biden.
It was not clear whether the Pentagon would hand over documents in response to a subpoena, or if Vice President Mike Pence would meet a request from the committees for related documents from his office.
But momentum appeared to build toward a showdown between the White House and Congress over their respective power as Democrats construct a case for removing Trump from power.
House Speaker Nancy Pelosi was due to speak to reporters at 6:30 pm (2230 GMT).
Earlier the House committees heard behind closed doors from George Kent, a State Department official who has voiced concern over the administration's efforts to pressure Ukraine into investigating Biden.
Kent was the fifth witness to be deposed after a whistleblower from the intelligence community filed a complaint that Trump had illegally sought foreign help for his reelection campaign and endangered national security in doing so.
On Monday former White House aide Fiona Hill testified during a 10-hour deposition that Giuliani was running a shadow foreign policy vis-a-vis Ukraine in order to help the president, according to several US media outlets.
Democratic congressman Eric Swalwell told CNN Hill's deposition had bolstered the argument for removing Trump, and said his party would be swift and "surgical" in building the case.
"Every arrow continues to point in the same direction," he said.
But Trump accused Democrats of hiding the investigation behind closed doors, saying on Twitter: "Democrats are allowing no transparency at the Witch Hunt hearings.
"Let the facts come out from the charade of people, most of whom I do not know, they are interviewing for 9 hours each, not selective leaks," Trump fumed.
Democrats are seeking information related to Trump pressing Ukraine to uncover dirt on White House contender Biden, while allegedly conditioning almost $400 million in US military aid on that favor.
The New York Times reported that Hill said her then-boss, former national security advisor John Bolton, grew alarmed by Giuliani's efforts, in conjunction with White House chief-of-staff Mick Mulvaney, to pressure Ukraine.
"I am not part of whatever drug deal Rudy and Mulvaney are cooking up," Bolton said, according to Hill's testimony as reported by the Times.
She is said to have testified that Bolton warned that Giuliani was "a hand grenade who's going to blow everyone up."
Senior diplomats to testify
Tuesday's appearance by Kent, the Deputy Assistant Secretary of State for European and Eurasian Affairs, was due to be followed Wednesday by testimony from former senior diplomat Michael McKinley, who resigned last week.
On Thursday investigators will hear from US ambassador to the European Union Gordon Sondland, a Trump appointee who allegedly worked with Giuliani to pressure Kiev.
The Democrats defended keeping the testimonies secret for now.
"We are going to continue to fill the picture in and very shortly release the transcripts to the American people and then decide where do we go from there," said Swalwell.
Democrat Mark Pocan told Wisconsin Public Radio that articles of impeachment could be drawn up by the end of the year.
"We already have a motive, we have a crime, we have a confession and we have evidence," he said.
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