Watergate reporter Carl Bernstein said Sunday that Republicans are coming to a "much greater consensus" that President TrumpDonald John TrumpRepublicans consider skipping witnesses in Trump impeachment trial Bombshell Afghanistan report bolsters calls for end to 'forever wars' Lawmakers dismiss Chinese retaliatory threat to US tech MORE is unfit to be president following the resignation of Defense Secretary James MattisJames Norman MattisThreatening foreign states with sanctions can backfire Overnight Defense: Erdoğan gets earful from GOP senators | Amazon to challenge Pentagon cloud contract decision in court | Lawmakers under pressure to pass benefits fix for military families Amazon to challenge Pentagon's 'war cloud' decision in federal court 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.