Let's imagine that Wall Street investment banker and long-time Social Security foe Peter Peterson had $1 billion in government bonds (also known as "IOUs"). Suppose that he decided to sell them. According to Glenn Kessler, the Washington Post's fact checker, this would create a burden for the U.S. government.
This sale of bonds would displace other bonds that the United States might want to sell in the financial market. This would lead to higher interest rates on U.S. debt. Therefore Mr. Peterson is contributing to our deficit problem.
That may seem more than a little silly to readers, which it is. Yet, this is the same way in which Kessler says that Social Security will be creating a fiscal burden. The program has bought $2.6 trillion in government bonds which are part of the $14.3 trillion debt subject to the debt ceiling. It will be relying on the interest from these bonds to pay for some benefits for the next decade, just as Mr. Peterson may use interest from government bonds that he holds to pay for his living expenses or funding his anti-Social Security agenda.
After 2022 the program will begin selling off its bonds. This will have the same effect on the market as if Mr. Peterson were selling his bonds. In Peterson's case he will directly sell his bond into the market, in the case of the Social Security program it will sell a bond to the government which will have to get the money by selling a new bond in the market (unless it raised taxes or cut spending to cover the price of the bonds).
Kessler also gets wrong the baseline for the projected longer-term shortfall for Social Security. After 2036 the program is projected to only have enough money to pay a bit less than 80 percent of scheduled benefits. However, if the law is never changed, then the program would only pay the benefits that could be financed through incoming Social Security tax revenue. The general fund would not be tapped to cover the shortfall.
Of course Congress could change the law, but budget debates usually start from the law as written, not as some individual might imagine it will be changed in the future. In this sense, it is 100 percent accurate to say that Social Security does not now nor will it in the future contribute to the deficit. Congress could change the law so at some point it does contribute to the deficit, but that is just a guessing game, not the current reality.