In order to gain 99% of the network hash rate, you need to increase the current rate by a factor of 100, and you would need enough mining equipment to generate 10,000,000,000 GH/s. At $2 per GH/s, that would cost you $20 billion.

The U.S. government has the ability to spend that much money, but keep in mind that the cost of such a project would be twice the cost of the Defense Department's largest program. Approval for such a project will never happen unless Bitcoin is perceived by many as a major threat to the U.S.