Eoin Higgins is the author of “Owned: How Tech Billionaires on the Right Bought the Loudest Voice on the Left.” Maine oysterman-turned-politician Graham Platner has been drawing consistently packed crowds across the rural state for months as he aims to take on longtime incumbent Republican Susan Collins in this year’s Senate race. He’s regularly outpolling his only other viable competitor for the Democratic nomination, Gov. Janet Mills. At 41, he could hold a seat for decades that Democrats have long had their eyes on. Since Mills joined the race last fall (Platner announced he was running that August), her support...