As a follow-up to Krugman's article, this is something I wrote in January of 2009:
...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. Maybe you can give it more gas along the way if needed without spinning out, and perhaps you can hold your position if you don't make it to the top, and then start again from the higher level, but that's not a chance I want to take when I'm sitting at the bottom wondering if I can make it to the top without wrecking my car -- the possibility of falling all the way back to the bottom and ending up worse off would make me want to start with sufficient momentum and then some. Essentially, I am arguing that there are crucial economic and psychological "tipping points" that must be reached in order for the economic recovery package to be effective (or at least, there's enough of a chance that they exist that they cannot be ignored when formulating robust policy). ...
Paul Krugman added:
I’d add that there may also be a political tipping point: if the stimulus package is too weak, conservatives will pile on after it fails to deliver, claiming that the whole concept has been discredited.