Mankiw riddles Krugman this:

Over the past eight years, Paul has tried to convince his readers that
Republicans are stupid and venal. History suggests that Republicans
will run the government about half the time. Does he really want to
turn control of healthcare half the time over to a group that he
considers stupid and venal?

A priori, it’s a tough question.  Empirically, though, it’s undaunting.  Republicans might be a little more inclined to hold down the rate of growth of Medicare.  But since its passage, both parties have run the program almost identically.  The same holds for other big, popular tent poles of the welfare state like Social Security.  If history is any guide, all Democrats need to do to ensure permanent bipartisan support for Obamacare is pass it.  As Roosevelt put it, “no damn politician can ever scrap my
social security program.”

Notice: In other areas, especially foreign policy, things don’t work this way.  Bush II invaded Iraq and winked at torture; Gore probably wouldn’t have.  Clinton invaded Haiti; Bush I probably wouldn’t have.

One can note these differences without being able to explain them.  But I think I’ve got a good explanation.  Here goes:

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.