The beauty of being a Keynesian is that one can never be proven wrong.
Paul Krugman explains why the real problem with the economy is that the government is not spending enough and should spend, spend, spend because only the government can save us from disaster. What happens if the government listens to Krugman and we travel even further down
The Road to Serfdom while the economy implodes even further? If you are Krugman, the obvious answer is that you didn't spend nearly enough. "If only you would have listened to my wisdom," he will say, "we could have been saved."
Krugman's theory has been tried before. In France. Read the (free) ebook
Fiat Money Inflation in France to see where that ended. Government stimulus at first appeared to work great (sound familiar) then, like gasoline on a fire the effect died away. What to do? More stimulus! This also appeared to work (only for a shorter amount of time), then, guess what, no more gasoline. More stimulus! Even less effective. On and on it went until the government collapsed and Paul Krugman - oops, I mean William Law, author of the grand inflation - was run out of France. This is where Krugman's theories would lead us.
Of course, since Mr. Krugman has a nice house on a Caribbean island and spends his days drinking Mai Tai's on the beach while telling us how to destroy ourselves, he probably will not mind being run out of the country.