This article is more than 7 months old

This article is more than 7 months old

Donald Trump’s threat to assert executive privilege in his impeachment defense to try to curb testimony by his former national security adviser John Bolton is an attempt to silence a key witness and could undermine constitutional principles, ex-justice department officials and legal scholars warn.

Bolton, a controversial figure whom a former colleague has testified had grave doubts about Trump’s pressure tactics to get Ukraine to investigate his potential 2020 rival Joe Biden, has indicated he would testify before the impeachment trial if he were subpoenaed.

Trump’s move comes after his absolute immunity claims during the House impeachment inquiry, which helped thwart testimony by other current and former top aides, such as the ex-White House counsel Don McGahn, sparking court challenges and a federal judge’s ruling that “presidents are not kings”.

Trump’s hardball legal tactics in the House prompted the second impeachment article of obstruction of Congress. The lead impeachment manager, the congressman Adam Schiff, said last Thursday: “In the history of the republic, no president has ever ordered the complete defiance of an impeachment inquiry,” stressing that nine administration officials had not complied with House subpoenas.

The key impeachment article charges Trump with abusing his office to benefit his 2020 campaign by secretly withholding almost $400m in military aid to Ukraine and a Washington DC meeting with its president, Volodymyr Zelenskiy, unless Kyiv publicly announced an inquiry into the former vice-president Joe Biden, and his son Hunter’s past ties to a Ukrainian energy firm.

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In their legal rebuttal to the impeachment charges, Trump’s lawyers said he “unequivocally denies each and every allegation in both articles of impeachment”.

But the prospect of Bolton’s testimony seems to unnerve Trump. In a Fox News interview, Trump said invoking executive privilege if Bolton testified would be “for the sake of future presidents”, a move that could curb, but not block entirely, potentially harmful testimony by Bolton.

In Davos on Wednesday, Trump told reporters that Bolton testimony would raise some “national security” problems, noting too that Bolton had departed “probably” not “on the best of terms … You don’t like people testifying when they didn’t leave on good terms.”

Whether Bolton testifies won’t be clear unless a majority of the Senate votes to allow witnesses, which most of Trump’s Republican allies are expected to oppose. Four GOP members would have to join Democrats for new witnesses to testify.

Ex-justice department officials say Trump has escalated his use of dubious legal tactics, including claims of absolute immunity and executive privilege, in impeachment battles.

“What Trump is doing is a wholesale attempt to silence key witnesses who might have relevant testimony,” said Joshua Geltzer, who was a senior justice lawyer in the area of national security during the Obama administration and is a visiting professor at Georgetown University Law Center.

Other ex-prosecutors share Geltzer’s concerns about Trump’s executive privilege threats.

“The idea that a president would invoke executive privilege to block his own impeachment is pretty close to frivolous,” said Paul Rosenzweig, an ex-prosecutor who was a senior counsel on Ken Starr’s team during the Clinton impeachment. “It shows the extent to which Trump is willing to go to assert executive privilege that hasn’t really existed before.”

Rosenzweig, now a senior fellow on national and cybersecurity at the non-partisan R Street Institute, added: “If the Senate lets him get away with it, [that] would be the ultimate example of undermining separation of powers, and a new example of Republican cravenness.”

Facebook Twitter Pinterest A protester holds a sign outside the impeachment trial on Thursday. Photograph: Erik S Lesser/EPA

Pressure for more witnesses has been increasing.

The House intelligence committee recently released a trove of incriminating documents and text messages from Lev Parnas, an indicted ally of Trump’s personal lawyer Rudy Giuliani who worked to dig up dirt on the Bidens and pressure Zelenskiy to investigate their Ukraine roles. Parnas, a Ukrainian American, recently told Rachel Maddow that Trump had been “aware” of everything and “knew exactly what was going on”.

But even if Trump asserts executive privilege, that wouldn’t stop Bolton or others from answering certain questions that might hurt Trump, says the ex-federal prosecutor Michael Zeldin.

For instance, if Bolton was asked why he said Giuliani was a “hand grenade who’s going to blow everybody up”, as House testimony by a colleague indicated, “assertion of executive privilege should not prevent him from answering”, Zeldin said.

A few moderate GOP senators, led by Susan Collins, seem to be moving to support witnesses. Collins said last Thursday that she would “likely” support calling witnesses, and she has reportedly been talking to four like-minded senators, enhancing the chances of some new witness testimony.

“The White House would be wasting its time to meet with and lobby those four senators against calling witnesses or issuing subpoenas,” said the veteran Republican operative Charlie Black.

The Washington Post reported on Monday that the Senate majority leader, Mitch McConnell, had been talking to Trump’s lawyers about a controversial fallback plan, if Bolton testifies: questioning could take place in secret to limit public attention.

Such a move in an impeachment trial could backfire legally, said Rosenzweig. “To classify Bolton’s testimony solely to conceal Trump’s violations of law, is itself a violation of law,” he said.