SALT LAKE CITY — The progressive activists who gathered in Utah two weeks ago to strategize for the midterm elections could each recall a moment when they realized the Democratic Party was their foe and decided to quit it. For Kenneth Mejia, 27, who ran for Congress as a Democratic write-in candidate two years ago, it happened when the party declined to include policies that inspire him — like single payer health care and a ban on fracking — in its 2016 party platform. For Diane Moxley, 49, who canvassed for Barack Obama in 2008, it was when the Occupy...