What Krugman said:
Well, there actually is a large constituency in America for a political leader who is willing to take responsible positions — to call for more investment in the nation’s education and infrastructure, to propose bringing down the long-run deficit through a combination of spending cuts and tax increases. And there is in fact a political leader ready and willing (maybe too willing) to play that role; his name is Barack Obama.
So why Americans Elect? Because there exists in America a small class of professional centrists, whose stock in trade is denouncing the extremists in both parties and calling for a middle ground. And this class cannot, as a professional matter, admit that there already is a centrist party in America, the Democrats — that the extremism they decry is all coming from one side of the political fence. Because if they admitted that, they’d just be moderate Democrats, with no holier-than-thou pedestal to stand on.
Concise, accurate, and to the point.