This article is more than 10 months old

This article is more than 10 months old

As the impeachment inquiry continued to cast dark clouds over the White House, Donald Trump said he was preparing to release details of a second call with Ukrainian president Volodymyr Zelenskiy.

Mick Mulvaney: new testimony draws Trump chief of staff into Ukraine scandal Read more

“We have another transcript coming out which is very important,” Trump told reporters at Joint Base Andrews before leaving for Alabama and a college football game. “I will give you a second transcript, because I had two calls with the president of Ukraine.”

The impeachment inquiry centres on a 25 July call between Trump and Zelenskiy in which, according to an incomplete memo released by the White House and repeatedly and incorrectly referred to by Trump as a “perfect” transcript, the US president asked his Ukrainian counterpart to “do us a favour”.

Testimony released this week has painted a damning picture of attempts to persuade Ukraine to investigate former vice-president Joe Biden and his son Hunter, potentially in return for an unfreezing of nearly $400m in military aid and a White House meeting with Trump.

The 25 July call and concerns over its content became known when an intelligence official filed an anonymous report. Trump and allies such as the Kentucky senator Rand Paul have led Republican pressure for the media to name the whistleblower, contrary to federal law.

On Saturday, House Republicans said Hunter Biden and the whistleblower should be called to testify in public hearings set to begin next week.

Adam Schiff, the Democratic chair of the House intelligence committee, reportedly said he was “evaluating” the list but added that the inquiry “will not serve … as a vehicle [for] the same sham investigations into the Bidens or 2016 that the president pressed Ukraine to conduct for his personal political benefit”.

Trump said his new “transcript” would probably be released on Tuesday. He also characterised the impeachment inquiry as “a lot of witch-hunt” but said Republicans defending him “have never been so united”.

Trump's phone call with Ukraine's president: read the full memorandum Read more

On Friday night, acting White House chief of staff Mick Mulvaney asked to join a lawsuit that seeks a ruling on whether senior advisers to Trump must testify in the impeachment inquiry.

The move came on the same day Mulvaney became the latest senior White House aide not to comply with a subpoena to appear before Democrat-led House committees receiving testimony in private. The suit he wants to join was filed in October by Charles Kupperman, former deputy to former national security adviser John Bolton.

House Democrats have not served a subpoena to Bolton, whose lawyer said on Friday he had information about “many relevant meetings and conversations”. Bolton has warned he will seek legal recourse if a subpoena is sent. In a game of legal chess, Kupperman’s subpoena has been withdrawn.

White House counsel Pat Cipollone has claimed senior aides do not have to comply with impeachment inquiry subpoenas, a claim that is fiercely contested. But career officials from the National Security Council and state department have testified in private.

Mulvaney found himself in the crosshairs after admitting last month that aid to Ukraine was held up and telling reporters to “get over it”, an admission he sought to retract.

This week, transcripts of testimony by White House aides linked Mulvaney to offers of a Trump meeting for Zelenskiy, if he agreed to investigate the Bidens and a conspiracy theory which says Ukraine, not Russia, intervened in the 2016 US election.

In a filing quoted by the Washington Post, Mulvaney’s lawyers said the issue of whether senior aides should testify “go[es] to the heart of our representative government and its promise to secure individual liberty by dividing the awesome power of government amongst itself”.

“Mr Mulvaney, like Mr Kupperman, finds himself caught in that division, trapped between the commands of two of its co-equal branches – with one of those branches threatening him with contempt. He turns to this court for aid.”

When Kupperman’s suit was filed, many observers said it was merely a delaying tactic. Critics of Cipollone’s position say it disregards the oversight role of Congress as set out in the constitution.

Facebook Twitter Pinterest Mick Mulvaney arrives in Columbia, South Carolina, in October. Photograph: Jim Watson/AFP via Getty Images

Speaking to the Guardian on Friday, Frank O Bowman III, author of High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump and a professor at the University of Missouri school of law, said the White House “gag rule” on senior advisers testifying was “completely abnormal”.

Bowman said: “Inasmuch as the no-shows are in response to presidential orders or strong admonitions, they amount to obstruction of the House’s constitutional impeachment function and are therefore a free-standing ground for impeachment.”

On Saturday, in a letter to Schiff, ranking Republican intelligence committee member Devin Nunes called for the whistleblower to testify and claimed: “Americans see through this sham impeachment process.”

Trump's attacks on whistleblower could do lasting damage to system, experts say Read more

Other individuals Nunes said should testify included Devon Archer, an associate of Hunter Biden; former Democratic National Committee staffer Alexandra Chalupa; and Nellie Ohr, a “former contractor for opposition research firm Fusion GPS”, which commissioned the famous Steele dossier on links between Trump and Russia.

The Wall Street Journal, meanwhile, reported that two Ukrainian Americans entangled in the impeachment inquiry pushed previous Ukrainian president Petro Poroshenko to investigate the Bidens and the conspiracy theory regarding 2016.

Lev Parnas and Igor Fruman worked with Rudy Giuliani, the former New York mayor commonly described as Trump’s personal attorney, on approaches to Ukraine. Parnas and Fruman have been charged with campaign finance offences. Giuliani has defied congressional demands for relevant documentation.

According to released testimony, Giuliani’s role in apparent extra-governmental foreign policy caused alarm among national security staffers.

Fiona Hill, a British-born Russia expert, told the committees Bolton characterised Giuliani as “a hand grenade who’s going to blow everybody up”.

According to Hill, Bolton also said he was “not part of whatever drug deal [EU ambassador Gordon] Sondland and Mulvaney are cooking up”.