Watergate reporter Carl Bernstein said Sunday that Republicans are coming to a "much greater consensus" that President TrumpDonald John TrumpIvanka Trump, Jared Kusher's lawyer threatens to sue Lincoln Project over Times Square billboards Facebook, Twitter CEOs to testify before Senate Judiciary Committee on Nov. 17 Sanders hits back at Trump's attack on 'socialized medicine' MORE is unfit to be president following the resignation of Defense Secretary James MattisJames Norman MattisNearly 300 more former national security officials sign Biden endorsement letter John Kelly called Trump 'the most flawed person' he's ever met: report Biden courts veterans amid fallout from Trump military controversies MORE.

Bernstein, appearing on CNN's "Reliable Sources," said there is concern that Trump is "unfit on psychological grounds" as well as unfit "because of his contempt for the law" and because of his "conduct of foreign policy."

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“What the Mattis letter has done in a monumental way is to push Republicans into making some real judgments. They’re talking to each other, there is coming to be a much greater consensus that he is unfit to be the President of the United States," Bernstein said. 

Mattis resigned last week from his position as secretary of Defense following Trump's decision to remove U.S. troops from Syria. Mattis wrote in his resignation letter that Trump deserves "a Secretary of Defense whose views are better aligned with yours."

He also expressed concern over Trump's views on traditional U.S. alliances.

"While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies," Mattis wrote in the letter.