TORONTO — HERE’S the puzzle of America today: the plutocrats have never been richer, and their economic power continues to grow, but the populists, the wilder the better, are taking over. The rise of the political extremes is most evident, of course, in the domination of the Republican Party by the Tea Party and in the astonishing ability of this small group to shut down the American government. But the centrists are losing out in more genteel political battles on the left, too — that is the story of Bill de Blasio’s dark-horse surge to the mayoralty in New York, and of the Democratic president’s inability to push through his choice to run the Federal Reserve, Lawrence H. Summers.