A conservative firebrand. A well-connected Democrat hoping for a tough midterm year for President Donald Trump. And a test of both major parties’ enthusiasm as an independent threat looms and a costly fight over a U.S. Senate seat dominate the airwaves. Political observers are braced for all of that as Maine’s race for governor is now poised for the general election. Republican lawyer Bobby Charles faces off against former state House Speaker Hannah Pingree, the Democratic nominee, while independent state Sen. Rick Bennett aims to spoil both sides’ paths to the Blaine House. Charles has fired up his base in part by painting the...