In late July, he told the country's new president, Volodymyr Zelensky, that Ukraine could improve its reputation and its "interaction" with the United States by investigating corruption, according to a Ukrainian government summary of the call. An intelligence watchdog has said the claims are "urgent" and "serious", but the acting top intelligence official has declined to provide them to Congress.
The Trump administration is declining to comment on reports that the whistleblower, whose identity has not been disclosed, is an intelligence officer detailed to the National Security Council and was authorized to listen in on the call or have access to its transcript. The Associated Press has not confirmed the reports.
The Democrats' investigation was launched earlier this month, before revelations that an intelligence official had lodged a complaint with the inspector general. "There is an urgency here that I think the courts will recognize", the California Democrat said.
The Ukrainian government's readout of the call said Mr. Trump was "convinced that the new Ukrainian government will be able to quickly improve image of Ukraine, complete investigation of corruption cases, which inhibited the interaction between Ukraine and the U.S.".
Trump tapped Maguire, a former Navy official, as acting intelligence director in August, after the departure of Director Dan Coats, a former Republican senator who often clashed with the president, and the retirement of Sue Gordon, a career professional in the No. 2 position.
Rep. Adam Schiff says it's also "deeply disturbing" that the White House appears to know more about the whistleblower's complaint than the complaint's intended recipient, Congress.
The information "deserves a thorough investigation", Schiff said.
The White House only last week gave up its hold on $250 million in military assistance to Ukraine, under pressure from lawmakers from both parties.
The president said Friday that 'it doesn't matter what i discussed, ' and insisted that 'someone ought to look into Joe Biden'. Giuliani initially said, "No, actually I didn't", but seconds later he said, "Of course I did".
Lawmakers seek answers after intel whistle-blower said to flag Trump
The complaint was filed with Atkinson's office on August 12, a date on which Trump was at his golf resort in New Jersey. Adam Schiff , D-Calif., has demanded Maguire's testimony and a copy of the complaint.
We got answers to the fact that this is, for example, the first time that a director of national intelligence has ever withheld a complaint from Congress.
The general counsel for the Office of the Director of National Intelligence, Jason Klitenic, responded to the subpoena on September 17, reiterating that the agency's refusal to hand over the complaint is to protect the whistleblower.
Maguire has refused to discuss details but he has been subpoenaed by the panel and is expected to testify publicly about the whistleblower complaint next Thursday.
The row has thrust Mr Trump's use of presidential powers and his determination to win the November 2020 presidential election into the spotlight.
Giuliani, explaining the contradiction, said he had asked Ukraine "to look into the allegations that related to my client, which tangentially involved Joe Biden in a massive bribery scheme".
Cuomo asked Giuliani, "Did you ask the Ukraine to investigate Joe Biden?". His agency's general counsel wrote a letter stating the complaint did not meet the ICWPA definition because it involved conduct "from someone outside the intel community and did not relate to intelligence activity", according to a report by Fox News. If the speculation is accurate, Maguire's actions, and the legal defense of those actions by Office of the Director of National Intelligence (ODNI) General Counsel Jason Klitenic, may have a firmer legal foundation than has so far been apparent.
Three Democratic panel heads - Eliot Engel (foreign affairs), Adam Schiff (intelligence) and Elijah Cummings (oversight) - said Mr Trump and Mr Giuliani had attempted "to manipulate the Ukrainian justice system to benefit the president's re-election campaign and target a possible political opponent".
The chairman said he would go to court, if necessary, to try to force the administration to turn over the information in the complaint. He merely could have said he was reviewing the matter and would have more to say later.