A series of high-stakes primaries unfolding across Maine, Iowa, Michigan, and Minnesota is testing his once-formidable machinery.

Democratic critics of Senate Minority Leader Chuck Schumer say his ill-fated backing of Maine Gov. Janet Mills shows he should stay out of the way in pending primaries.

Democrats’ hopes of reclaiming the U.S. Senate are colliding with a fight within their own party. In Maine, Senate Minority Leader Chuck Schumer has thrown his weight behind Gov. Janet Mills in a ...