Watergate reporter Carl Bernstein said Sunday that Republicans are coming to a "much greater consensus" that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE is unfit to be president following the resignation of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November 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.

"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,” @carlbernstein says of Trump. pic.twitter.com/oc0YAcZ1bz — Reliable Sources (@ReliableSources) December 23, 2018

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.