These people have no idea what they are describing. If the Social Security payroll taxes are invested in US Treasury debt (as they are), they fund other government departments by definition. You can't say that a given president has borrowed money from the trust fund and now should pay it back. The tax hikes on banks that the authors advocate will also be immediately invested in US Treasury debt, and therefore automatically lent out to the operating branches of the government.
By the way, I can buy US Treasuries myself through my account at Fidelity. Indeed, I can submit a non-competitive bid at the auctions of new issues and Fidelity adds zero mark up. Nobody needs the Social Security Administration to invest in US Treasuries if they want to. If the government stopped taking Social Security payroll taxes you'd be free to use that money to buy US Treasuries. This talk of putting our savings at the mercy of Wall Street is nonsense. Read the Article at HuffingtonPost