The leader of the impeachment investigation of President Donald Trump said Tuesday that witnesses had provided substantial support for allegations that Trump illegally tried to force Ukraine to investigate his political rival Joe Biden.

While the White House and Trump's personal lawyer Rudy Giuliani refused to turn over subpoenaed documents on the Ukraine affair to Congress, House Intelligence Committee Chairman Adam Schiff said five witnesses had backed up allegations of misuse of power that could underpin formal impeachment charges.

Witnesses have supplied information showing that Trump's July 25 phone call with Ukraine leader Volodymyr Zelensky, in which Trump appeared to tie military aid to getting dirt on Biden from Kiev, was planned in advance and then followed up, Schiff said.

‘We've made dramatic progress in answering some of the questions surrounding that July phone call... in which the president of the United States sought to coerce a vulnerable ally into conducting what can best be described as sham investigations involving his opponent,’ Schiff told reporters.

‘We have learned that call was not in isolation. There was a great deal of preparatory work that was done before the call. There was a lot of follow-up work done after the call,’ he said.

- Ukraine pressure like a 'drug deal' -

Schiff spoke a day after former White House Russia expert Fiona Hill reportedly told Congressional investigators in closed-door testimony that several high-level aides reported the Trump-Zelinsky phone call to a White House lawyer as possible wrongdoing by the president.

Hill said her own boss, then-national security advisor John Bolton, characterized Trump's pressure on Zelensky together with Giuliani and White House chief of staff Mick Mulvaney as akin to a narcotics transaction.

‘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 New York Times.

Hill said Bolton also warned that Giuliani, who is reportedly under federal investigation over his Ukraine dealings, is ‘a hand grenade who's going to blow everyone up.’

- Out of the loop -

House Democrats on Tuesday interviewed a top State Department official responsible for Ukraine, George Kent, about his role between the White House, Giuliani and Kiev, and prepared to question three more diplomats this week.

Kent told lawmakers that Mulvaney arranged for him to be cut out of decision making on Ukraine in favor of three officials close to Trump, The Washington Post reported, particularly his energy secretary, ambassador to the European Union and special envoy to Ukraine.

Meanwhile on Twitter, Trump lashed out at lawmakers, saying ‘Democrats are allowing no transparency at the Witch Hunt hearings.’

Democrats are seeking to prove Trump sought foreign aid to boost his reelection next year, violating US election laws.

Trump has admitted asking Zelensky for help to investigate Biden, but said it was for legitimate suspicions of corruption and not political reasons.

The White House meanwhile Tuesday questioned the investigation's legitimacy by refusing to turn over budget office, Pentagon and vice presidential documents related to the Ukraine dealings.

And Giuliani brushed off a subpoena, branding the inquiry ‘illegitimate’ and ‘unconstitutional,’ as he risked being declared in contempt of Congress.

- Schiff: White House 'stonewalling' -

Schiff accused the White House of stonewalling the investigation and said that could be added to impeachment charges.

‘The case for obstruction of Congress continues to build,’ he said.

Schiff said the committees involved in the inquiry would continue to collect evidence in private depositions, but were prepared to release the testimony once it was all complete.

Democrats involved in the process suggested that articles of impeachment could come by the end of the year.

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.

‘We already have a motive, we have a crime, we have a confession and we have evidence,’ Democrat Mark Pocan told Wisconsin Public Radio.

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