Megan McArdle tries to shift the debate now:
Chait, and others writing in this vein, refute the strongest claims of the supply-side movement: that tax cuts produce astonishing growth, or that cutting taxes can increase tax revenue. Then they imply that they have thereby refuted all the economic claims in favor of tax cuts, which they haven't, not even close.
I don't think this is what anyone's doing. Rather, what Chait's doing -- and what I'm doing here -- is noting that there is this huge grotesque error lurking at the heart of the Republican Party's political agenda. Brendan Nyhan has a big ole list of instances of Bush and Cheney citing the notion that tax cuts will pay for themselves if there's still any doubt as to the centrality of this notion. Meanwhile, the reason people like Jon and I and other liberals spend so much time pointing out that this claim is false is precisely the same as the reason conservatives spend so much time defending it: it's an extremely potent political claim.
There's a systematic effort by the right to convince people that tax cuts are not merely beneficial in some ways or beneficial all things considered but that there are actually no tradeoffs whatsoever. Getting that idea taken seriously in the press is very powerful politically, so those of us who don't approve of the tax cutting policy agenda are very upset about the ability of conservatives to get away with making it, over and over and over again.
Meanwhile, Megan's comparison of this phenomenon to the idea that Bill Clinton has been known to, for example, overstate the role of Urban Empowerment Zones in spurring the economic growth of the 1990s is a little say. The point about the supply siders isn't that politicians sometimes lie. The point is that a vast superstructure has grown up around this particular lie. Most national leaders in the Republican Party subscribe to it. Those who don't, meanwhile, keep quiet about it. The major conservative opinion publications propagate it, as do the conservative talk shows on radio and cable, as do many conservative newspaper columnists, and the major conservative think tanks.
This is a weird phenomenon. If Hillary Clinton got up at the next presidential debate and said "I believe a policy of 'Medicare for all' could save enough money to pay for a universal preschool program and more generous Social Security benefits," Barack Obama would say she was out of her mind, major liberal commentators would agree, and if she started angrily defending the claim against all comers it would be big trouble for her campaign. By contrast, were Mitt Romney to attack John McCain's embrace of supply-side dogma, that would swiftly destroy Romney's campaign as all the major institutions of the right moved to expel him from the movement.