Ooooh, I really like this Krugman blog post called The Cracked Conservative Mirror,

whenever you read conservatives trying to critique what they think the other side believes, you find them assuming that their opponents must be mirror images of themselves. The right believes that less government spending is always good, regardless of circumstances, so it assumes that the other side must always favor more government spending. The right says that deficits are always evil (unless they’re caused by tax cuts), so they assume that the center-left must favor deficits in all conditions.

Krugman’s assumption is that the conservatives are actually naive and not listening to the arguments of their opponents, whereas the more cynical assumption would be that they intentionally setup straw man arguments to mislead their constituents. Following Hanlon’s razor, Krugman’s probably right.