More than 1,000 former US justice department officials, including some of the top government lawyers in the country, have called on attorney general William Barr to resign in the wake of the Roger Stone scandal.

Some 1,143 alumni of the Department of Justice posted to Medium on Sunday a group letter that tore into Barr for “doing the president’s personal bidding” in imposing on prosecutors the recommendation of a reduced sentence for Stone, a longtime friend of Donald Trump who was convicted of lying to and obstructing Congress and threatening a witness in the Russia investigation.

Barr, the officials said, had damaged the reputation of the department for “integrity and the rule of law”.

The searing letter is the latest twist in a rapidly spiraling constitutional crisis that began earlier this week when Barr imposed his new sentencing memo, slashing a seven- to nine-year proposed prison term suggested by career prosecutors. In the fallout, the four prosecutors who had handled the case resigned in disgust.

The letter carries weight because its signatories are exclusively drawn from past DoJ public servants. Among them are several former US attorneys appointed by both Republican and Democratic presidents and section chiefs of key elements of the justice department including its antiterrorism unit.

They write that it is unheard of for top leaders of the justice department to overrule line prosecutors in order to give preferential treatment to close associates of the president. They say that amounts to political interference that is “anathema to the department’s core mission and to its sacred obligation to ensure equal justice under the law”.

Barr’s action amounted to an existential threat to the republic, the former officials say: “Governments that use the enormous power of law enforcement to punish their enemies and reward their allies are not constitutional republics; they are autocracies.”

Barr tried to squash the perception he had been leaned on by Trump by calling on the president to stop tweeting about criminal prosecutions. He told ABC News such unrestrained comments were “making it impossible for me to do my job”.

But speculation continued to swirl that Barr had kowtowed to the president. Demoralisation spread rapidly through the DoJ, intensifying when it emerged that Barr has ordered outside prosecutors to re-examine criminal cases against Trump associates including former national security adviser Michael Flynn.

The president thinks Andy McCabe should have been punished because he lied and lied several times to the investigators Kellyanne Conway

Despite palpable distress among both serving and former officials, and multiple warnings that Trump and Barr are threatening the very rule of law, the White House has continued to inflame the situation. Trump counsellor Kellyanne Conway on Sunday claimed the president was a victim of a “two-tier criminal justice system” that was actively undermining him and his associates.

Conway used Fox News Sunday to pour fuel on the fire. The truth, she claimed, was that far from making a dangerous intervention in criminal cases involving his friends and perceived enemies, Trump himself is the victim of the politicisation of the justice system.

“If you’re President Trump or people associated with him there’s prosecutions that have gone one way,” Conway said, alluding to the original sentence recommended for Stone which she contrasted with the decision announced by the justice department on Friday to drop charges against former FBI deputy director Andrew McCabe.

Directly contradicting her own claim that Trump, despite his “vast powers”, was not engaging in political interference in criminal cases, Conway proceeded to interfere in a criminal case. She called McCabe a “serial liar and leaker” and went on: “The president thinks that Andy McCabe should have been punished because he lied and lied several times to the investigators.”

McCabe, a deputy to fired FBI director James Comey and a key figure in the Russia investigation, was fired by Trump in March 2018, two days shy of retirement.

Trump and Barr seen in the White House in November. Photograph: Nicholas Kamm/AFP via Getty Images

The furore over Trump ignoring protocols that have kept a distance between the White House and federal prosecutors since Watergate began when the president slammed the proposed sentence for Stone as “horrible and very unfair”. Hours later, Barr announced that he was imposing a reduced recommended sentence.

Trump then made the constitutionally dubious claim that as president he has the “legal right” to stick his finger into any criminal case.

On Saturday he duly re-entered the fray over McCabe, claiming falsely that DoJ inspector general Michael Horowitz recommended the former FBI man’s firing. Horowitz referred criticisms of McCabe to prosecutors but did not recommend dismissal.

On Sunday Marc Short, chief of staff to vice-president Mike Pence, made further contentious comments on CNN’s State of the Union. Like Conway, he claimed without evidence that criminal justice was skewed against the president.

“The scales of justice aren’t balanced any more,” he said, “when someone like Roger Stone gets a prosecution that suggests a nine-year jail sentence and candidly someone like Andy McCabe who also lied to federal investigators gets a lucrative contract here at CNN. People say, ‘How is this fair?’ and that’s the source of the president’s frustration.”

The row has also become a major talking point among Democrats vying to take on Trump in November. Former vice-president Joe Biden told NBC’s Meet the Press: “No one, no one, including Richard Nixon, has weaponised the Department of Justice” as much as Trump.

The crisis is personal for Biden, given the efforts to coerce Ukraine into investigating him and his son Hunter which led to Trump’s impeachment. Last week it was revealed that Barr has set up a channel to review information gathered in Ukraine by Trump lawyer Rudy Giuliani relating to the Bidens.

“To have a thug like Rudy Giuliani reporting to the attorney general – I mean this is, this is almost like a really bad sitcom,” Biden said.

“Any self-respecting Republican or Democratic top-flight lawyer would have just resigned by now, in my view. It’s just the things that are being done are so beyond the pale.”