PORTLAND, Maine (AP) — A week ago, Maine Democrat Zak Ringelstein wasn't quite ready to consider himself a member of the Democratic Socialists of America, even if he appreciated the organization's values and endorsement in his bid to become a U.S. senator.

Three days later, he told The Associated Press it was time to join up. He's now the only major-party Senate candidate in the nation to be a dues-paying democratic socialist.

Ringelstein's leap is the latest evidence of a nationwide surge in the strength and popularity of an organization that, until recently, operated on the fringes of the li...