Yes, what?

Nice. Over at The Conscience of a Liberal Krugman asks:

I see, however, that some commenters are declaring that it’s all OK because of the voodoo “dynamic” effects of tax cuts for the wealthy. I guess my question is, what on earth would make anyone believe in that old nonsense at this point?

I mean, we’ve had two fairly clear-cut tests in recent decades: the Clinton tax hike, which all the usual suspects said would produce disaster, and the Bush cuts, which would supposedly produce a wonderful boom. How did those turn out?

Also, the Romers have some careful new research looking at older data — the drastic tax swings of the interwar years — and find only small effects.

So let me rephrase my question: what conceivable evidence would convince people that supply-side magic doesn’t work?