In a simple median voter model, it never matters which party is in control. Whoever runs the government does as the median voter commands.
In the real world, however, politicians often have some political slack – a range of electorally safe options. They can use this slack is all kinds of ways: To pursue their own vision, to sell out to special interests, to cheat on their wives, whatever.
Question: Where does slack come from? The main answer: Slack exists insofar as the median voter is roughly indifferent. (Imperfect information, contrary to conventional wisdom, does not imply slack; rationally ignorant voters could easily discipline politicians with Beckerian punishment strategies). Politicians can safely do A instead of B as long as voters are – out of apathy or deference – indifferent between them.
OK, so what’s the difference between health policy and foreign policy? For health policy, the median voter has fairly specific preferences. He knows he likes giving free medicine to the American elderly, he knows he hates rationing, and he knows he doesn’t want to listen to fiscal Cassandras. Hence, the political juggernaut that is Medicare. For foreign policy, in contrast, the median voter has a big range of indifference. If the President says we need to invade Iraq, he’ll go along with it for a couple years at least; if the President says Iraq isn’t a problem, the median voter will go along with that, too.
Bottom line: Libertarians should fear government-run health care no matter who’s in charge. For liberals, however, it doesn’t make much difference. As long as public opinion is firmly on your side, it doesn’t matter who runs the government – venality and stupidity notwithstanding.
This analysis makes sense to me. Notwithstanding the huge downside to health care reform there is an aura of inevitability – unless voters start to understand the downside.