Politics in Maine are complicated, which means Ms. Collins faces criticism no matter what she does. Independent voters — “unenrolled,” in the Maine lexicon — account for the largest percentage of the electorate; these are the voters she must win. Democratic registration is growing, a problem for Ms. Collins. Just as problematic are the Trump Republicans who do not care much for their senior senator.

“She stabbed the Republican Party in the back,” growled Arthur L. Carter, 86, a retired Army major wearing a “proud American” T-shirt. “She hasn’t really supported our president.”

Amy Fried, a professor of political science at the University of Maine, noted the changing landscape: “I’m looking at CNN exit polls from 2014 — 37 percent of liberals voted for Collins and it has 39 percent of Democrats voting for Collins,” she said. “It’s hard to imagine that that’s going to happen again.”

More than anything else, Ms. Collins’s vote to confirm Justice Kavanaugh to the Supreme Court shifted that dynamic.