Democrats have privately grown worried about Kamala Harris’s standing among working-class voters in the crucial “blue-wall” states—particularly in Michigan. Donald Trump has assiduously courted union members and noncollege-educated white voters with a message focused on high costs, manufacturing and the threat of China to the U.S. economy. Senior Democrats, including Michigan Gov. Gretchen Whitmer, want a sharper economic appeal from Harris and have conveyed those...