Krugman quotes a stimulus analogy by Mark Thoma:
I think the stimulus package is like driving up an icy hill. If you don’t have enough momentum from the start and fail to provide enough “stimulus” to get the car over the crest of the hill, you can slide all the way back to the bottom, crashing into things along the way and ending up worse off than when you started.
As of right now, I agree that the president-elect hasn't floored it enough to make it to the top. However, I'm confident that he will. Krugman adds:
...if the stimulus package is too weak, conservatives will pile on after it fails to deliver, claiming that the whole concept has been discredited.
Of course this would mean an affirmation of Reaganomics -- even though it's been a proven failure.