Former White House counsel to former President Nixon, John Dean, ripped President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on Sunday, saying he doubts Trump has any idea what White House counsel Donald McGahn has told special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE.

Dean's comments come amid reports that McGahn is cooperating with Mueller’s investigation into possible ties between the Trump campaign and Russia and possible obstruction of justice by the president.

“I doubt you have ANY IDEA what McGahn has told Mueller,” Dean tweeted in response to an earlier Twitter thread from Trump bashing The New York Times. “Also, Nixon knew I was meeting with prosecutors, b/c I told him. However, he didn’t think I would tell them the truth!”

@realDonaldTrump (I still have trouble using the title Mr. President for someone installed by Putin), I doubt you have ANY IDEA what McGahn has told Mueller. Also, Nixon knew I was meeting with prosecutors, b/c I told him. However, he didn’t think I would tell them the truth! — John Dean (@JohnWDean) August 19, 2018

Trump tweeted earlier Sunday morning that The New York Times, which first reported McGahn's cooperation with the special counsel, was reporting a "fake" story that he said implied McGahn must be a “John Dean type 'RAT.'”

On Saturday, the Times reported that McGahn gave over 30 hours of testimony to investigators, talking to them extensively about episodes including Trump’s firing of former FBI Director James Comey and the president's repeated prompting of Attorney General Jeff Sessions to claim oversight of the special counsel, despite his recusal from Mueller's Russia probes.

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The Times said McGahn had grown concerned that the president was preparing to set him up as a scapegoat over the issue of possible obstruction of justice, prompting him to provide as much information as possible to Mueller's investigation.

McGahn and his lawyer, William Burck, have reportedly been doing as much as possible to cooperate with Mueller since last year. The Times reported that McGahn told investigators Trump tried to assert control of the Russia investigation, “giving investigators a mix of information both potentially damaging and favorable to the president.”

Trump said later Saturday that he allowed McGahn “and all others to testify” and that he has “nothing to hide.”

Dean became a key witness for prosecutors in the Watergate scandal after pleading guilty to a count of obstruction of justice. He said Saturday that “McGahn is doing right!” in response to the Times report.