Republican officials say privately that the performance of the economy under Mr. Trump has not been a major motivating factor for pro-Trump voters. For some Americans on the right, it may even be contributing to the mood of political apathy that has so alarmed G.O.P. leaders, since voters who are optimistic rarely vote with the intensity of those who are angry or afraid.

America First Action, a political committee aligned with Mr. Trump, conducted a series of focus groups over the summer and concluded the party had a severe voter-turnout problem, brought on in part by contentment about the economy and a refusal by Republicans to believe that Democrats could actually win the midterm elections.

Conservative-leaning voters in the study routinely dismissed the possibility of a Democratic wave election, with some describing the prospect as “fake news,” said an official familiar with the research, who spoke on condition of anonymity because the data was not intended to be disclosed. Breaking that attitude of complacency is now the Republicans’ top priority, far more than wooing moderates with gentler messaging about economic growth.