Republican pollsters have reinforced the concerns of the party’s congressional leaders that the White House is not always helping when it comes to the midterms.

“They have spent way too much time yelling about investigations and not enough time educating about the economy,” Frank Luntz, a longtime Republican pollster, said in an interview. The president, he added, “deserves a lot more credit than he has been given, but he’s also somewhat responsible because he hasn’t been so focused and attentive to the changes that have really happened” in the economy.

Neil Newhouse, another pollster, has made clear his worries that too many Republicans are taking the midterms for granted.

“We need to make real the threat that Democrats have a good shot of winning control of Congress,” he said in a presentation for the Republican National Committee at the end of August that was recently shared with the president.

Voters who are not die-hard Trump supporters may not “believe there’s anything at stake in this election,” Mr. Newhouse wrote. “Put simply, they don’t believe that Democrats will win the House. (Why should they believe the same prognosticators that told them that Hillary was going to be elected president?).”

Mr. Trump has alternately acknowledged to aides and supporters that the climate is troublesome and insisted that the worst will never happen. It is not clear that he actually believes his talk of a “red wave,” or if he is trying to will it into existence, advisers and allies say.

But the potential for an electoral disaster on Nov. 6 has certainly not been lost on White House officials, who say it has added to existing anxiety about what will come next in the president’s term. According to interviews with over half a dozen current and former White House aides and campaign officials, they are girding for the possibility of a political landscape — and, possibly, a West Wing — that could look very different after Election Day.