But, listening to strategists and voters in a critical state for Democrats, the midterms feel like a long time ago. Instead, there are widespread worries that the momentum in Pennsylvania, and in other key Rust Belt states, could screech to a halt if the issues in the 2020 presidential primaries and the party’s eventual nominee stray too far left for the region’s many centrist voters.

“The more we have presidential candidates or newly elected congresspeople talking about the Green New Deal, talking about ‘Medicare for all,’ talking about socialism, the more that plays into the Trump campaign’s hands,” said Ed Rendell, a former Pennsylvania governor and national Democratic chairman.

He mentioned issues that are tantalizing to some primary voters and candidates but which risk alienating general election voters, such as reparations for descendants of slaves and a rapid, costly transition to carbon-free energy. “Reparations? What are we talking about?” Mr. Rendell scoffed. “Having only renewable energy by 2030? It’s not possible to achieve that.”