The party’s challenge was crystallized last week in a Quinnipiac survey of voters in Pennsylvania, one of the states that helped Mr. Trump win in 2016. The poll found that 77 percent of voters described their own financial situation as “excellent” or “good” — but that Mr. Trump would lose there by 11 percentage points against Joseph R. Biden Jr., one of the leading Democratic candidates.

Mr. Trump’s low approval ratings, which are at odds with normal ratings for a president in a humming economy, also point to the deep divisions in the country. The president’s erratic conduct and his gut instinct for issues of culture and identity, combined with the leftist energy in the Democratic Party and the chance that the Supreme Court could reconsider Roe v. Wade, will most likely further polarize an electorate that already cleaves along racial, gender and class lines when it comes to Mr. Trump.

“We’re pulling further apart, not together, and the traditional issues are being eclipsed — because if ‘peace and prosperity’ worked, there would still be a Republican majority in the House,” said Representative Tom Cole, an Oklahoma Republican, who envisioned “an impending clash out there with the two sides mobilized and demonizing the opposite side.”

It is, Mr. Cole added, “a long way from Ronald Reagan and ‘Morning in America.’”

Mr. Trump’s re-election campaign and congressional Republicans surely will highlight the country’s economic gains should they continue through 2020, of course, and will target Democrats over issues such as taxes and the size of government — particularly if a liberal like Senator Bernie Sanders or Senator Elizabeth Warren emerges as the Democratic nominee.

But both parties have overwhelming incentives to push next year’s election toward issues of the heart, not the head.