President Obama will no doubt clash regularly with his newly empowered partisan adversaries in Congress. But the most important struggle in U.S. politics over the next two years will be inside the Republican Party. And the person who can play a decisive role in that battle is Jeb Bush. Yes, Democrats have their own divides, usually described as differences between their populist and Wall Street camps. Still, there is broad agreement inside the party, even among most of its Wall Streeters, that wage stagnation and economic inequality are major problems for the country and that Democrats need to make dealing...