Harry Reid, who has led Senate Democrats since 2005, is shrewd, temperamental and highly partisan. But as the 76-year-old Nevadan nears his retirement after next month’s elections, he has grown sentimental. Mr. Reid wants to leave the Senate in Democratic hands. “To cap his career,” the Washington Post’s Ben Terris wrote in a September retrospective, “Reid wants to leave the Senate better than he found it.”

Democrats have at least an even chance of making Mr. Reid’s first wish come true. Republicans hold a 54-vote majority...