The case for Mr. Ryan is that he best exemplifies the nature and stakes of this election. More than any other politician, the House Budget Chairman has defined those stakes well as a generational choice about the role of government and whether America will once again become a growth economy or sink into interest-group dominated decline.

Against the advice of every Beltway bedwetter, he has put entitlement reform at the center of the public agenda—before it becomes a crisis that requires savage cuts. And he has done so as part of a larger vision that stresses tax reform for faster growth, spending restraint to prevent a Greek-like budget fate, and a Jack Kemp-like belief in opportunity for all. He represents the GOP’s new generation of reformers that includes such Governors as Louisiana’s Bobby Jindal and New Jersey’s Chris Christie.

As important, Mr. Ryan can make his case in a reasonable and unthreatening way. He doesn’t get mad, or at least he doesn’t show it. Like Reagan, he has a basic cheerfulness and Midwestern equanimity.