In an extraordinary rebuke of a president of his own party, Mr. Corker said he was alarmed about a president who acts “like he’s doing ‘The Apprentice’ or something.”

“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…

“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.”