“American electoral politics suffers from wildly unrealistic expectations about the power of elected officials — especially the president — to determine the nation’s economic performance. The American economy is not managed in any real sense: It is, by and large, a free-market system operating within a complex global marketplace. Of the forces that affect our economy, far more are outside the president’s control than are within it. And to the extent a president can shape the economy, the effects of his decisions will often become apparent only after he leaves office.”