PORTLAND, Maine (AP) — Last Friday, Maine Democrat Zak Ringelstein wasn’t ready to consider himself a formal member of the Democratic Socialists of America, even if he appreciated the organization’s values and endorsement in his bid to become a United States senator. Three days later, he told The Associated Press that was ready to become the only major-party Senate candidate in the nation to be a dues-paying democratic socialist. The swift evolution is latest evidence of a nationwide surge in the strength and popularity of an organization that, until recently, operated on the fringes of the liberal movement’s farthest left...