Ryan is trying to start a conversation on the desirable role and limits of government. He’s trying to make it possible to talk about sensitive issues — mainly Social Security and Medicare — without being vilified. President Obama recognized that when he called Ryan’s plan a “serious proposal.” But since then, Democrats have resorted to ritualistic denunciations of him as pillaging Social Security and Medicare. Legitimate debate becomes impossible. If Democrats don’t like Ryan’s vision, the proper response is to design and defend their own plan. The fact that they don’t have one is a national embarrassment.