Hundreds of former federal prosecutors – and counting – signed an open letter published on Monday expressing their belief that Donald Trump would have faced “multiple felony charges of obstruction of justice” if he were not president.

Multiple aspects of Trump’s conduct described in a report of the Trump-Russia investigation submitted in March by special counsel Robert Mueller were probably criminal, the prosecutors write. Mueller had declined to weigh whether to charge Trump, citing justice department guidelines prohibiting the indictment of a sitting president.

But the letter’s signatories, which include prominent Republicans from administrations going back to Richard Nixon, said it was clear that Trump would have faced charges had he not been protected by the guidelines.

“Each of us believes that the conduct of President Trump described in Special Counsel Robert Mueller’s report would, in the case of any other person not covered by the Office of Legal Counsel policy against indicting a sitting President, result in multiple felony charges for obstruction of justice,” the prosecutors wrote in part.

Trump has claimed that the Mueller report was a “complete exoneration” of his conduct. But the prosecutors outline three main areas in which they said that conduct warranted criminal charges:

The president’s efforts to fire Mueller and to falsify evidence about that effort

The president’s efforts to limit the scope of Mueller’s investigation to exclude his conduct

The president’s efforts to prevent witnesses from cooperating with investigators investigating him and his campaign

The signatories include former prosecutors at all levels, from line attorneys to US attorneys to special prosecutors to former senior officials in the Department of Justice. The letter was originally published with more than 300 signatures but had more than 450 by Monday afternoon.

“Proud to be on this nonpartisan list of hundreds of former federal prosecutors who would have charged Trump with obstruction based on the evidence in Mueller’s report,” said Elie Honig, a former federal prosecutor from the southern district of New York. “It’s not even a close call for me.”

The attorney general, William Barr, and his deputies in the justice department decided there was “not sufficient” evidence to charge Trump.

But Barr’s expressed reasoning for not charging Trump was undercut by critics who said he was acting in defense of the president instead of the rule of law. They pointed out that Barr originally mischaracterized Mueller’s reasoning for not charging Trump himself; Barr had claimed that the guideline against indicting a sitting president was not a major factor in Mueller’s decision, when Mueller states explicitly that it was.

Mueller wrote Barr a forceful letter in March stating that a separate letter in which Barr claimed to summarize his report in fact “did not fully capture the context, nature and substance of this Office’s work and conclusions”.

“There is now public confusion about critical aspects of the results of our investigation,” Mueller wrote.

Some of that confusion might be cleared up later this month, when Mueller is expected to testify before Congress. Trump has discouraged the special counsel from appearing.

“Bob Mueller should not testify,” Trump tweeted on Sunday. “No redos for Dems!”