“He concerns me,” Mr. Corker added. “He would have to concern anyone who cares about our nation.”

Mr. Trump poses such an acute risk, the senator said, that a coterie of senior administration officials must protect him from his own instincts. “I know for a fact that every single day at the White House, it’s a situation of trying to contain him,” Mr. Corker said in a telephone interview.

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“A lot of people think that there is some kind of ‘good cop, bad cop’ act underway, but that’s just not true,” Mr. Corker said.

Without offering specifics, he said Mr. Trump had repeatedly undermined diplomacy with his Twitter fingers. “I know he has hurt, in several instances, he’s hurt us as it relates to negotiations that were underway by tweeting things out,” Mr. Corker said.

All but inviting his colleagues to join him in speaking out about the president, Mr. Corker said his concerns about Mr. Trump were shared by nearly every Senate Republican.

“Look, except for a few people, the vast majority of our caucus understands what we’re dealing with here,” he said, adding that “of course they understand the volatility that we’re dealing with and the tremendous amount of work that it takes by people around him to keep him in the middle of the road.”

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“I don’t know why the president tweets out things that are not true,” he said. “You know he does it, everyone knows he does it, but he does.”

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Mr. Corker would not directly answer when asked whether he thought Mr. Trump was fit for the presidency. But he did say that the commander in chief was not fully aware of the power of his office.

“I don’t think he appreciates that when the president of the United States speaks and says the things that he does, the impact that it has around the world, especially in the region that he’s addressing,” he said. “And so, yeah, it’s concerning to me.”