Eight years after Donald Trump shattered the Blue Wall, some Democrats worry he could do it again.

Most polls show Kamala Harris tied or leading Trump in the critical battleground states of Pennsylvania, Michigan and Wisconsin after a commanding performance at the debate. Party leaders are projecting confidence publicly. But on the ground, jittery elected officials, strategists and allies are quietly pointing to warning signs for the vice president.

The Teamsters withholding an endorsement from Harris this week — after internal polling showed most respondents backing Trump — is sparking fresh concerns that the GOP nominee could have higher-than-expected support among union members, especially men. Labor leaders in other sectors attest that, like in 2016 and 2020, the former president has maintained a grip on key parts of their rank-and-file despite his anti-union record. Privately, Democrats say Harris still has work to do to win over older, white, working-class voters who make up a large portion of the electorate in the Rust Belt and have been hit by high prices.