An independent poll of Maine’s U.S. Senate race released earlier this month showed that nearly a third of respondents said they did not know enough about Graham Platner to say whether they like him. It was a small sample of just 810 voters, but the Pan Atlantic Research survey revealed a key vulnerability for Platner, a 41-year-old oyster farmer who is leading Gov. Janet Mills in recent polls for the Democratic primary. “Platner’s central issue is that no matter how much buzz he’s got online among Democratic likely voters, he’s still, to a lot of Mainers, sort of a mystery,”...