House Democrats are weighing extraordinary steps to secure testimony from a whistleblower whose complaint prompted an impeachment inquiry, masking his identity to prevent Donald Trump’s congressional allies from exposing the individual, according to three officials familiar with the deliberations.

The steps under consideration include having the whistleblower testify from a remote location and obscuring the individual’s appearance and voice, these officials said.

The efforts reflect the deepening distrust between Democrats running the impeachment inquiry of Mr Trump and their GOP colleagues they see as fully invested in defending a president who has attacked the whistleblower’s credibility and demanded absolute loyalty from Republicans.

“There are lots of different protocols and procedures we’re looking into to find out what works and doesn’t work to protect the identity of the whistleblower,” said an official familiar with the talks. “That is paramount.”

The officials spoke on the condition of anonymity to speak frankly.

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Negotiations over the whistleblower’s testimony come as House Democrats are trying to unearth as much evidence as possible before they move towards impeachment.

The whistleblower’s complaint centred on Trump’s 25 July call pressing Ukrainian president Volodymyr Zelensky to investigate former vice president Joe Biden, a 2020 Democratic presidential candidate, and his son Hunter.

On Tuesday, the House Intelligence Committee will question Gordon Sondland, the US ambassador to the European Union.

The whistleblower reported that Mr Sondland met with Mr Zelensky to give “advice” about how to “navigate” Mr Trump’s demands, working behind the scenes to carry out the president’s wishes in a country that’s not a member of the European Union.

In text messages provided to Congress, Mr Sondland insisted that Mr Trump’s decision to withhold nearly $400m (£325m) in military aid to Ukraine was not a quid pro quo – as diplomat Bill Taylor had feared.

On Monday, Democrats subpoenaed defence secretary Mark Esper and Office of Management and Budget acting director Russell Vought for documents related to the withholding of US military aid from Ukraine.

“The committees are investigating the extent to which President Trump jeopardised national security by pressing Ukraine to interfere with our 2020 election and by withholding military assistance provided by Congress to help Ukraine counter Russian aggression, as well as any efforts to cover up these matters,” lawmakers said in a letter.

Signing the letter were intelligence committee chairman Adam Schiff, oversight chairman Elijah Cummings, and foreign affairs chairman Eliot Engel.

Mr Trump told his acting chief of staff, Mick Mulvaney, to hold back the military aid for Ukraine shortly before his July call.

Mr Trump has repeatedly denied that there was a “quid pro quo” between the military assistance and the request to investigate the Bidens.

At the White House on Monday, Mr Trump lashed out at Democrats over their impeachment inquiry.

“You can’t impeach a president for doing a great job ... This is a scam,” he said at an event on trade with Japan.

Mr Trump and other administration officials have repeatedly rejected requests for information and testimony in the Democrats’ investigations.

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The state department ignored a subpoena for documents on Friday, and an attorney for two of Mr Giuliani’s associates who work on the Ukraine matter, Lev Parnas and Igor Fruman, rebuffed another request Monday.

“ ... Overly broad and unduly burdensome,” John Dowd, who is representing Mr Parnas and Mr Fruman, wrote to House investigators.

“This, in combination with requiring immediate responses, leads me to the inescapable conclusion that the Democratic committee members’ intent is to harass, intimidate and embarrass my clients.”

The highly unusual arrangement for the whistleblower’s testimony underscores the toxicity between Republicans and Democrats on the House Intelligence Committee, which was once considered among the most bipartisan but is now spearheading the divisive impeachment investigation.

One individual familiar with the discussions said this is the first time the panel has had to take such extraordinary measures to protect a witness – including from the individual’s own GOP colleagues.

In a further sign of the breakdown of comity, the committee majority restricted access to the visitor logbook after GOP staffers leaked names of individuals signing in for job interviews when the majority was hiring new staff in early 2019, according to a committee aide who spoke on the condition of anonymity to freely describe the situation.

Of the whistleblower, Mr Trump has said he wants to meet his “accuser” and has warned of “big consequences”.

Mr Trump even alluded to the death penalty for individuals who gave information to the whistleblower, likening them to spies and outraging longtime whistleblower advocates who called his comments chilling.

“In addition, I want to meet not only my accuser, who presented second and thirdhand information, but also the person who illegally gave this information, which was largely incorrect, to the ‘whistleblower’,” Mr Trump tweeted.

“Was this person spying on the US president? Big consequences!”

(Statista/Wikimedia (Statista/Wikimedia)

Democrats are concerned that without such rare precautions for the whistleblower, Republicans on the House Intelligence Committee could learn and then leak the name of the individual, who has agreed to answer questions before the intelligence committees in the House and Senate.

Andrew Bakaj, a lawyer representing the whistleblower, sent a letter to acting director of national intelligence Joseph Maguire expressing fears for his client’s safety.

Mr Bakaj also noted that “certain individuals” have issued a $50,000 “bounty” for “any information” relating to his client’s identity.

House Democratic chairmen, meanwhile, immediately cried foul and accused Mr Trump of “witness intimidation” – even as some of his staunchest Capitol Hill allies also began pushing for the disclosure of the whistleblower’s identity.

On Sunday, Senate Judiciary Committee chairman Lindsey Graham said that if the House impeaches Mr Trump, “I will make sure” the whistleblower faces public questioning.

“If the whistleblower’s allegations are turned into an impeachment article, it’s imperative that the whistleblower be interviewed in public, under oath, and cross-examined,” Mr Graham said on Fox News.

In the House, Democrats overseeing the logistics of the testimony for the House impeachment inquiry are leaning towards a staff-only session that would prevent lawmakers from attending and asking questions, according to officials familiar with the conversations.

Aides have even considered having the whistleblower testify from a separate location via a video hookup in which the camera would obscure the whistleblower’s image and alter his voice, possibly with modification technology.

Two other possibilities include having the whistleblower sit behind a screen or partition, or conducting audio-only testimony.

“Schiff does not want to burn his identity,” a senior congressional official said.

Details for the whistleblower’s testimony are still being worked out, the officials said. Little can be finalised until security clearances for the whistleblower’s lawyers are resolved so they can begin more substantive discussions about logistics, they said.

Mark Zaid, one of three lawyers for the whistleblower, said that as of Monday, two have their clearances; the third needs only to sign documents.

“It’s just a logistics issue,” Mr Zaid said. “It will happen over the next couple of days.”

The whistleblower’s legal team is also in discussions with the Senate Intelligence Committee about testifying, according to one individual familiar with the talks.

The Senate panel, headed by Richard Burr and Mark Warner, is known for a more bipartisan approach; its staffers are not considering the extraordinary measures being discussed by the House.

The House and Senate committees are expected to meet separately with the whistleblower – both likely away from the Capitol.

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The off-campus meetings probably will be in a secure room provided by the executive branch, according to officials. The meetings could take place “within the next couple of weeks”, but no date has been set, one official said.

Should the whistleblower testify from another location, the House panel could ask Intelligence Community inspector general Michael Atkinson to confirm the identity of the whistleblower.

Mr Atkinson, who adjudicated the whistleblower’s complaint in August, determined that the whistleblower’s allegations were credible and constituted an “urgent” concern to national security.

Congressional aides have been discussing the possibility of a transcript from the testimony, but whether the whistleblower’s legal team agrees to that depends on whether the individual’s identity can be protected.

The lawyers would not want information shared that could identify the whistleblower.

The legal team has been hired by a second witness, one who had firsthand knowledge of the events in the complaint.

So far, there has been no discussion with Capitol Hill about that person speaking with Congress. That would require permission from the director of national intelligence, but that’s “pro forma” at this point, an official said.