Kamala Harris campaigned Tuesday with Michigan Senate nominee Abdul El-Sayed, predicting that the progressive will become a “national voice” during a joint appearance between two Democrats in a state crucial to the party’s fortunes in this fall’s midterms — and the presidential primary to follow.

Former Vice President Kamala Harris will campaign on Tuesday in Michigan in an effort to rally Democratic voters behind progressive Senate candidate Abdul El-Sayed in a swing state that will help dete...