The Defense Budget and Political Will

Beyond the substantively mysterious nature of the decision to exempt “security” spending from a freeze, I wonder about the politics. Spencer Ackerman wrote earlier today about the defense spending conundrum:
Now, if you read through Harrison’s paper, you’ll see it contains a key assumption. Because defense spending is so bloated, and the deficit so big and the economy so bad, then obviously defense spending has to drop, so it makes sense to reprioritize what’s actually in the national interest. But that assumes political will — both from Congress and from the Obama administration — that is absolutely not in evidence. And it also assumes countervailing political pressures — i.e., the desire not to be demagogued as weak on defense — that are in abundance will suddenly abate. So we’re left with … an unsustainable defense budget and spending freezes/cuts in for more politically vulnerable clients, like the poor and middle class.
Part of what’s odd about this is that while cutting defense spending isn’t popular, as best I can tell it’s one of the least-unpopular items out there:

Defense, homeland security, and foreign operations are all less popular than virtually everything in the “non-security discretionary” budget. And among the things that we spend a non-trivial amount of money on, defense is by far the least popular.
That’s not to say that politicians should blindly follow the public’s will. Cutting scientific research is a terrible idea, and the public’s desire to slash the State Department’s operational budget is bizarre. But as long as political stunts are happening, I don’t understand why public opinion doesn’t get more play.