The truth that no American politician will say is that they can't cut the budget because we the people won't let them.
John Sides at the Monkey cage illustrated a recent Economist/YouGov poll on what people are willing to cut - against what the government actually spends money on.
I suspect that some of this has to do with how the questions were asked, but I am also fairly certain that most citizens have very little idea where the government's budget goes. The elephant in the room is the defense budget. Right now we pay not just to raise and keep a military to fight definable threats, such as the very real wars in Afghanistan and Iraq (however questionable the strategy that got us there), we also pay for a lot of weapons, equipment, and capabilities based on wildly speculative definitions of "threat."
The military budget is bigger than the next 20 nation's budgets combined. Even cutting it in half would still leave with us a truly enormous military budget, and such serious cuts are going to have to be considered as the U.S. finds itself as one nation among others, as opposed to the power above all the rest. America will remain a mighty powerhouse, but economic reality will soon dictate that we cannot maintain this unquestioned hegemony for much longer.