Demography isn't destiny—and assuming that it is will likely make liberals overreach again. Many are arguing these days that President Obama has forged a new majority coalition of women, minorities, young people and upscale cultural liberals so large and durable that he can do what no president has done before—pursue a very liberal agenda without serious opposition or defections from his own party. Demography is destiny, this argument holds, and it is irrevocably on the side of Mr. Obama and the Democratic Party. Yes, there will be fewer whites and more minorities in the future, and Republicans will have to...