Democrats are scrambling in the wake of Graham Platner’s exit from the U.S. Senate race in Maine, but one of the questions the party should be asking itself is this: what have you learned from the fiasco?Platner, a military veteran and oyster farmer, was the darling of progressive leaders after Gov. Janet Mills dropped out of the race in late April. He was gruff, combative, and primary voters seemed to like that. That’s all progressive leaders needed. Sen. Elizabeth Warren gushed “He’s my kind of man,” a phrase that has aged like milk. Platner’s tattoo resembling a Nazi symbol? Pay...