During an interview on “The Late Show,” Tuesday, Senator Bernie Sanders declared socialist ideas to be mainstream, after host Stephen Colbert suggested it’s hurting Democrats at the ballot box. Mr. Colbert pointed out that most of the candidates Mr. Sanders and Alexandria Ocasio-Cortez campaigned for lost their primaries. “Why do you need to call yourself socialists? Because that’s freighted with so much negativity in the United States,” the late-night comedian asked. “Maybe there’s a little taint to socialism that turns people off?”