WASHINGTON — Lawyers for President Trump unleashed a sharp attack on former FBI director James Comey in a confidential memo last year to the special counsel, casting him as ‘‘Machiavellian,’’ dishonest, and ‘‘unbounded by law and regulation.’’

The letter appears to be an attempt to undermine the credibility of a law enforcement leader they see as a crucial witness against the president.

The letter, obtained by the Associated Press, provides a window into the formation of a legal strategy currently used by Trump’s lawyers as they seek to pit the president’s word against that of the former FBI director.

Comey’s firing in May 2017 helped set in motion the appointment of special counsel Robert Mueller, and one-on-one conversations with Trump that Comey documented in a series of memos helped form the basis of Mueller’s inquiry into whether the president obstructed justice.

Mueller is looking broadly into Russia’s meddling in the US election and its contacts with people in Trump’s campaign.

The June 27, 2017, letter was written by Marc Kasowitz, then the president’s lead lawyer, as Mueller and his team were in the early stages of their investigation into Trump associates and as they had begun examining whether the president, by firing Comey, had sought to stymie an FBI investigation into the Trump campaign’s ties to Russia.

The White House initially pointed as justification for the firing to a Justice Department memo that faulted Comey for his handling of the Hillary Clinton e-mail investigation, though Trump later said that ‘‘this Russia thing’’ was on his mind when he made the move.

The document also could have new relevance in the aftermath of a Justice Department inspector general report that criticized Comey for departing from established protocol in the Clinton investigation.

It’s not clear to what extent, if any, the attacks on Comey have resonated with Mueller’s team, which continues to seek an interview with the president on whether he had a corrupt intent when he fired the FBI director.

And even in the face of withering criticism, Comey has been largely consistent in his telling of his interactions with Trump in his memos, his book, and numerous press interviews he has given in recent months.

The 13-page document aims to identify for Mueller what the lawyers believe are grievous errors both in how Comey handled the Clinton investigation and in his early, and limited, encounters with the president.

In it, Kasowitz argues that Comey cannot be trusted as a witness because he repeatedly embellished his testimony before Congress, put his ‘‘own personal interests and emotions’’ above FBI protocol and left a cloud of undue suspicion above the president’s head.

‘‘Over the last year, Mr. Comey has engaged in a pattern of calculated unilateral action unbounded by governing law, regulation and practice, and plainly motivated by personal and political self-interest,’’ wrote Kasowitz, who has since stepped aside as lead lawyer.

Lawyers for Comey declined to comment Saturday. Kasowitz and Trump lawyer Jay Sekulow did not immediately return messages, and former Trump attorney John Dowd declined to comment.

The document does not focus on questions of Trump’s guilt or innocence. Instead, it casts in a negative light actions that Comey has said he carefully reasoned and that he has vigorously defended in his book and interviews.

Those include the decision to announce on his own without Justice Department consultation the conclusion of the Clinton investigation, and the decision months later to brief Trump — then the president-elect — on allegations about him in a salacious dossier.

‘‘Mr. Comey continued his Machiavellian behavior after President Trump was elected,’’ Kasowitz wrote.

Among the principal lines of attack are Comey’s acknowledgment that he provided his lawyers with contemporaneous memos about his interactions with Trump and authorized one of them to share details with the news media.

In one such encounter, Comey said the president asked him at a private dinner for his loyalty and that Comey offered him ‘‘honest loyalty’’ instead.

‘‘There is no ‘honest loyalty’ in an FBI Director surreptitiously leaking to civilians his privileged and confidential conversations with the president, or misappropriating and disseminating his confidential FBI memos or their contents about those meetings,’’ Kasowitz wrote.