An election-night party in the Brooklyn, N.Y., district where congressional candidate Claire Valdez won the June Democratic primary.Insurgent Democratic socialist candidates are knocking off establishment-backed incumbents in primaries this year, powered by voters drawn to populist promises such as universal healthcare.At the same time, many centrist Democrats, fearful of the party’s leftward lunge, are fighting back and warning the Democratic socialists could doom the party in November and beyond.This collision faces its next test in August, when Democratic socialists are on the ballot in Missouri, Wisconsin and Michigan, including state Rep. Donavan McKinney’s bid to unseat Rep. Shri Thanedar in...