"You have to have large cuts in the rest of the non-defense budget, very large cuts," said Paul Van De Water, an analyst at the budget think tank. "Whether it's politically and practically achievable is subject to question."
Romney also is light on details on his tax cut proposal.
He says he wants to cut rates by 20 percent, but won't specify how he'll find the $5 trillion required to pay for it. For all the rhetoric of tax loopholes and cleaning up the tax code, finding that kind of money would require looking at popular deductions and tax breaks for the middle class.
Those include deductions for mortgage interest, charitable contributions and state and local taxes, and breaks for college savings, employer-paid health insurance and families with children.
Tax experts say he could very well come up short.