Money comes into the federal government through taxes and bonds. The vast majority of it is then spent on old-people programs, poor-people programs, and defense.
Mitt Romney is promising that taxes will go down, defense spending will go up, and old-people programs won’t change for this generation of retirees. So three of his four options for deficit reduction — taxes, old-people programs, and defense — are now either contributing to the deficit or are off-limits for the next decade.
Romney is also promising that he will pay for his tax cuts, pay for his defense spending, and reduce total federal spending by more than $6 trillion over the next 10 years. But the only big pot of money left to him is poor-people programs. So, by simple process of elimination, poor-people programs will have to be cut dramatically. There’s no other way to make those numbers work.