Maine Gov. Janet Mills suspended her U.S. Senate campaign Thursday morning, citing a lack of financial resources. That's the official explanation. The more accurate one is that the polls showed her trailing badly to Graham Platner, an oysterman from coastal Maine with no electoral experience. Mills had every structural advantage working for her: she’d already won a statewide election, had name identification, and the support of Senate Minority Leader Chuck Schumer. The writing was on the wall for weeks, but Mills’s exit from the race was her concession that all the momentum on the Democratic side was for Platner. Platner had long...