The Democratic incumbent was leading in the polls and favored to hang onto her seat. Her GOP opponent, Eric Hovde, CEO of a $3 billion bank, was on the defensive after tens of millions in Democratic attacks over his vast wealth and past comments. And she had a new nominee atop the Democratic ticket, with Kamala Harris injecting fresh energy into one of the nation’s most evenly divided battlegrounds. But then the GOP zeroed in on a new line of attack. In the run-up to Election Day, Republicans have put a spotlight on Baldwin’s same-sex relationship and the career of...