Washington Post authors Philip Rucker and John Wagner report that there’s concern among Clinton supporters who believe Trump might do well in three Rust Belt states: Michigan, Pennsylvania and Wisconsin.

“In Michigan, Pennsylvania and, to a lesser extent, Wisconsin, an affinity for Trump’s message of economic populism and nationalism has surprised many Democrats,” Rucker and Wagner wrote. “These are big, industrial states they have carried for the past three decades — and where Clinton, so far, has not fully focused.”

In Pennsylvania, Clinton and Trump are in a statistical tie, according to RealClearPolitics polling averages. In Michigan, Trump closed the gap between the two candidates considerably once he became the GOP nominee, and now trails Clinton by just 4 points (though the last poll was taken at the end of May).