Rudy Giuliani, the personal lawyer to Donald Trump, has been subpoenaed by House congressional committees to produce documents related to his and the US president’s attempt to coerce Ukraine into meddling in American democracy.

Giuliani, a former mayor of New York, has emerged as a pivotal figure in the Ukraine scandal that engulfed Trump in recent days and led Democrats to open an impeachment inquiry.

On Monday the Democratic chairs of the intelligence, foreign affairs and oversight committees issued a subpoena to Giuliani, requiring him to hand over materials relevant to the impeachment inquiry by 15 October.

“The committees are investigating the extent to which President Trump jeopardized national security by pressing Ukraine to interfere with our 2020 election,” their joint letter said. “Our inquiry includes an investigation of credible allegations that you acted as an agent of the President in a scheme to advance his personal political interests by abusing the power of the Office of the President.”

In response, Giuliani said the chairs have “prejudged” the case, and that the subpoena raises “significant issues concerning legitimacy”.

Meanwhile the New York Times reported that, in another recent phone call, Trump urged Australia’s prime minister, Scott Morrison, to help the attorney general, William Barr, discredit the Russia investigation by the special counsel Robert Mueller.

The White House restricted access to the call’s transcript to a small group of presidential aides, an unnamed official told the Times, which noted it was “an unusual decision that is similar to the handling of a July call with the Ukrainian president”.

Barr has also held private meetings overseas with foreign intelligence officials seeking their help in a justice department investigation that Trump hopes will discredit his own intelligence agencies’ conclusions regarding Russian interference in the 2016 election, the Washington Post reported.

“Barr has already made overtures to British intelligence officials, and last week the attorney general traveled to Italy, where he and [John] Durham [the US attorney in Connecticut who is reviewing US intelligence work around the 2016 election] met senior Italian government officials and Barr asked the Italians to assist Durham, according to one person familiar with the matter,” the paper said.

Giuliani has been operating outside the state department over the past year to push conspiracy theories about Ukraine. He has held meetings with Ukrainian officials in an attempt to push false allegations that the former vice-president Joe Biden compelled Ukraine to fire a prosecutor investigating his son, Hunter, who was on the board of a Ukrainian gas company.

There is no evidence Joe Biden or his son, Hunter, engaged in corruption in Ukraine.

The chairs of the three House committees noted that Giuliani has acknowledged his efforts to pressure Ukrainian officials on national TV.

“In addition to this stark admission, you stated more recently that you are in possession of evidence in the form of text messages, phone records and other communications indicating that you were not acting alone and that other Trump administration officials may have been involved in this scheme,” wrote the House intelligence committee chairman, Adam Schiff, the foreign affairs chairman, Eliot Engel, and the oversight chairman, Elijah Cummings.

Facebook Twitter Pinterest The New York Times reported that Trump urged Australia’s prime minister, Scott Morrison, to help the attorney general discredit the Russia investigation by special counsel Robert Mueller. Photograph: Mark Lyons/EPA

In his July phone call with the Ukrainian president, Volodymyr Zelenskiy, Trump pressured Ukraine to work with Giuliani and Barr to investigate Biden, a potential challenger to the president in next year’s election, triggering an intelligence community whistleblower’s complaint.

The Trump administration sought to restrict access to records of his conversation with Zelenskiy by putting them on a system used to hold classified material, the complaint said, adding the call did not contain any information pertinent to national security but was potentially embarrassing.

Zelenskiy said on Monday he does not plan to publish his own notes from the call. “There are certain nuances and things which I think it would be incorrect, even, to publish,” he told journalists at a military site near Kyiv, according to Reuters.

He dodged questions of whether Ukraine planned to pursue an investigation at Trump’s request into Biden or his son Hunter. “We are open, we are ready to investigate [but] it has nothing to do with me. Our independent law enforcement agencies are ready to investigate any case in which the law was broken.”

Play Video 1:54 Donald Trump says he is 'trying to find out about a whistleblower’ – video

For his part Trump, with false claims and incendiary language, continued to lash out at both Schiff, widely seen as the public face of the impeachment inquiry, and the whistleblower, whose identity remains secret.

The president zeroed in on criticism that Schiff received for his opening statement at a hearing last week in which he parodied Trump’s conversation with Zelenskiy.

Schiff “illegally made up a FAKE & terrible statement, pretended it to be mine as the most important part of my call to the Ukrainian President, and read it aloud to Congress and the American people”, Trump wrote. “It bore NO relationship to what I said on the call. Arrest for Treason?”

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The comment echoed Trump’s comment during a 2016 debate with Hillary Clinton that if he was in charge, she would “be in jail”. In addition, “Lock her up!” became a common chant at his rallies.

There is no basis for accusing Schiff of treason, which is defined by the constitution as waging war against the US or providing material support to one of its declared enemies, and is punishable by death.

Shooting the messenger is a familiar Trump tactic. But this time his words and actions are spelled out in a rough transcript and the whistleblower complaint. As his responses to the threat of impeachment become increasingly wild and wayward, he is receiving unusual criticism from some Republicans and former administration officials.

On Sunday, Trump rewrote a quotation from Pastor Robert Jeffress, who told Fox News: “If the Democrats are successful in removing the President from office (which they will never be), it will cause a Civil War like fracture in this Nation from which our Country will never heal.”

The Republican congressman Adam Kinzinger tweeted: “I have visited nations ravaged by civil war. I have never imagined such a quote to be repeated by a President. This is beyond repugnant.”

Facebook Twitter Pinterest Adam Schiff, the chair of the House intelligence committee. Photograph: Andrew Harnik/AP

Many analysts predict Trump will be impeached by the House. Mitch McConnell, the Senate majority leader, confirmed on Monday that the Senate would have “no choice” but to take the matter up.

Such is Trump’s grip over the Republican party and conservative media that conventional wisdom holds he would be acquitted by the Senate, maybe even receiving a political boost. But polls show growing public support for impeachment.

Speaking at the Brookings Institution in Washington on Monday, senior fellow Jon Hudak noted that Richard Nixon won re-election by a landslide in 1972 and had a job approval rating far higher than Trump’s.

Hudak said: “To think on election day 1972 that Richard Nixon would be impeached and removed from office, which but for his resignation he would have been, to think that would have happened in less than two years, people would have laughed in your face.

“But alas, the evidence was built, the tapes came out and movement happened within the Republican conference and the Senate. Granted, our politics is different now from what it was in 1974, but this idea that there is nothing that can come out that will move a Republican senator towards voting to convict the president I think is foolish.”

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Lawyers representing the whistleblower, whose identity remains secret, said they have “serious concerns for our client’s personal safety, as well as for others connected to this matter”.

Democrats on the Senate judiciary committee have written to its chairman, Lindsey Graham, urging that witnesses be protected from intimidation. “This committee should not sit idly by as the president threatens potential witnesses, whose testimony may be crucial to congressional investigations into credible allegations against him,” they said in a letter.