FROM THE HORSE'S MOUTH
If you don't believe me, and there's no reason that you should, then perhaps you will believe that nonmarketable Treasury securities represent double taxation when you hear it from your ex-president's mouth.
In the Analytical Perspectives section of his fiscal 2000 budget, William Jefferson Clinton said: “Trust fund bonds are not real economic assets that can be drawn down in the future to fund benefits. Instead, they are claims on the Treasury that, when redeemed, will have to be financed by raising taxes, borrowing from the public, or reducing benefits or other expenditures.”
The highlighted words mean, send a bigger check, borrow the money, or do without something else you've set aside or sent money for us to provide. We know how the government borrows money, don't we? With a credit card that we pay later.
Other statements from other sources:

Robert D. Reishauer, former head of the Congressional Budget Office, CBO.