I think Matt makes the wrong comparison here:
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.
If Clinton said that, heads would nod. A very strong argument could be made that administrative and bargaining savings -- i.e, the government saying they're going to pay 20 percent less for Lipitor, and Pfizer will just have to deal -- from a Medicare-for-All system would save enormous amounts of money. Obama wouldn't dare attack it, he'd just argue that he doesn't think it politically possible, and the sort of policies required for those savings have tradeoffs Americans may not want to make. (Incidentally, I don't think Medicare-for-All would create those savings, but not because it couldn't, only because we wouldn't want to implement the necessary regulations.)
Supply-siderism, by contrast, is a proven failure. Unlike how we can look to Canada's single-payer system and notice that they pay 50 cents on ever dollar we spend, you can look at the 90s, when the supply-siders promised gloom-and-doom if Clinton increased taxes, and say that their theories are definitively discredited. I'm trying to think of a policy as thoroughly whack, but as thoroughly adopted, on the left, but I'm not coming up with much. The correct political comparison to a Republican decrying supply siderism is if a Democrat said we needed to cut Social Security benefits -- he'd be destroyed by the party. Problem is, that's a bad idea Democrats would be rightly flayed for adopting, while sane economic concepts are good ideas Republicans would be wrong marginalized for admitting.
