As the presidential race enters the final stretch, politicians are recycling the usual cons to make people believe this election will be different. At last week’s Democratic National Convention, sham idealism had a starring role, accompanied by ritual denunciations of cynicism. But idealism has a worse record in Washington than a New Jersey senator. “Idealism is going to save the world,” President Woodrow Wilson proclaimed shortly after World War I left much of Europe in ruins and paved the way for communist and Nazi takeovers. Wilson’s blather provoked H.L. Mencken to declare that Americans...