McGahn absorbed most of Hurricane Donald's force after the president found glowing media coverage - following his well-received speech to a Joint session of Congress - crowded out by the Sessions mini-scandal.
Trump's remarks came after The New York Times reported that McGahn had "cooperated extensively" with the investigation by special counsel Robert Mueller, taking part in at least three interviews with investigators totaling 30 hours.
The Axios report calling the decision an "epic" mistake, states that within the White House several top officials believed - and continue to believe - that Trump made a huge mistake in the early days of Mueller's investigation into Russian meddling in the 2016 election. The president immediately reacted Saturday evening, saying that it was he who allowed McGahn and others to be interviewed and ordered "millions" of pages of documents to be provided. No Collusion, No Obstruction.
Special counsel Robert Mueller is investigating whether President Trump's campaign team colluded with Russian Federation to influence the outcome of the presidential election.
The Times said McGahn had shared detailed accounts about the episodes at the heart of the inquiry into whether Trump obstructed justice.
Trump, meanwhile, appears to have mistakenly believed that McGahn would act as his personal attorney who would defend the president's interests.
Don't miss out on the latest news and information. But he had drawn an analogy to the communist witch hunt once before - when he falsely claimed that President Barack Obama had tapped his phones in Trump Tower.
Ex CIA directors issue unprecedented Donald Trump condemnation
Experts have described Trump's move as an unprecedented effort to silence prominent detractors, with some likening it to Sen. Praising John Brennan's character, honesty, and integrity, he invited Trump to revoke his own clearance as well.
"Time for Mueller investigation to file report", Giuliani wrote on Twitter.
The Times reported that McGahn's decision to cooperate was partly due to the fact that the President's initial legal team had chose to fully cooperate with Mueller's investigation into Russian interference in the 2016 presidential election, believing their client had nothing to hide and they could bring a quick end to the probe.
Dean was White House counsel to Nixon during Watergate.
The counter-punch comes after days of criticism from former US national security officials and military leaders who've cast Trump's cancellation of Brennan's clearance - an apparent retaliation for his criticism of the president - as a unsafe precedent.
On Saturday, John Dean tweeted that McGahn is doing the right thing. Joseph McCarthy's search for Communists in the US government in the 1950s.
According to the Times, McGahn is cooperating in the investigation - potentially to the president's detriment - to avoid being taken down by the investigation himself.
"People always view cooperation as giving up incriminating information but you can cooperate, tell the truth and not give up incriminating information because there's none to give up", Giuliani said. But Mr. McGahn's lawyer has offered only a limited accounting of what Mr. McGahn told the investigators, according to two people close to the president.