There has been a void in the Republican presidential race. The party's candidates have spoken about immigration, taxes, social issues and the war in Iraq. Mitt Romney, Rudolph Giuliani and John McCain have also spoken frequently about Ronald Reagan in order to position themselves as the political heirs to the great president. The candidates, however, have overlooked a central idea that animated Mr. Reagan's view of government. That was federalism, the constitutional principle that the federal government's responsibilities are "few and defined" as James Madison put it. Mr. Reagan believed the federal government had grown too big and swallowed up...