Over dinner with the president and other Republican congressional leaders this month, Senator Mitch McConnell, Republican of Kentucky and the majority leader, phrased his advice for the president in the form of a reminder: Mr. Trump should never forget his central role in the 2018 campaign, Mr. McConnell said, explaining that Republicans’ prospects are linked to what he says and does and underscoring that their one-seat advantage in the Senate was in jeopardy.

If Mr. McConnell’s warning was not clear enough, Marc Short, the White House’s legislative liaison, used the dinner to offer an even starker assessment. The G.O.P.’s House majority is all but doomed, he said.

But Mr. Trump was not moved. “That’s not going to happen,” he said at different points during the evening, shrugging off the grim prognoses, according to multiple officials briefed on the conversation.

The disconnect between the president — a political novice whose confidence in his instincts was grandly rewarded in 2016 — and more traditional party leaders demonstrates the depth of the Republicans’ challenges in what is likely to be a punishing campaign year.