The center of gravity in American politics moved left in Tuesday’s off-year elections. Republicans took a big step back from the tea party. An ebullient progressive was elected mayor of New York City. And a Democrat was elected governor of Virginia after campaigning unapologetically as a supporter of gun control and a liberal on social issues. The one bright spot for Republicans, Chris Christie’s landslide reelection in New Jersey, was won precisely because Christie ran briskly away from the party’s right wing and developed a civil relationship with President Obama. His victory speech spoke of the need for politicians to...