Forgive me if I have this totally wrong, but as far as I understand it, Social Security doesn't actually contribute to the deficit. It's entirely self-financed, and is restricted from drawing money from outside it's own funds when it can't completely pay it's bills. From what I know of the program, it's literally forbidden by law from contributing to the deficit.
let me take a stab at explaining......
Social Security has a "trust fund" run by a board of directors, advised by a bunch of actuaries, that looks after the fund.
When SS started up in the 30s, the retirement age was 65, and average life expectancy was 61 years, for every person collecting SS benefits, there were about 9 people paying into the system.
As people started living longer, and also on average entering the workforce later in life due to more education, this ratio started to shift. Currently there about 2 folks paying in for every person collecting... the ratio shrinks to about 3 to 2 at the peak of the baby boom bulge getting into retirement.
In the early 80s they "fixed" social security by raising SS taxes and very modestly tweaking retirement ages in the future and a few other minor changes.
Because of this fix, SS actually ran a pretty healthy surplus in most of the 80s and 90s, where the money flowing into the SS trust exceeded the benefits being paid out.. the theory being that like good fund managers they were building up a surplus now in the knowledge that in out years their obligations would rise.... based upon these accumulated surpluses SS is, officially at least, solvent till about 2030 or so.. (this may have slightly changed due to the recent fiddling with payroll taxes in the stimulus package, bust tax cut extension, etc)
Where this ties into the federal deficit is that the single and only asset in the SS trust fund is special US Treasury bills, so the 2.5 trillion that "officially" exists in the trust fund to help offset rising costs as the baby boomers retire exists only to the degree that the US Treasury can actually redeem these Treasury bills to the SS fund.
SS is now pretty close to "breakeven" (I think very mildly negative IIRC) on a cash flow basis with revenues coming in matching payments pretty closely, but in out years the cash flow gets very negative and the SS fund will require the treasury to redeem these special T-Bills in order to have the cash to actually payout the benefits to the baby boon.
The effective consequence of this is that soon tens, and fairly soon hundreds of billions of dollars a year of the special T-Bills held by the SS Trust fund will need to be redeemed by the treasure to fund SS benefits... this need to replay these T-Bills is what, on a cash flow basis, will massively contribute to the deficit faced by the federal government.
The short and dirty skinny on this is that the SS fund saved +/- 2.5 trillion dollars to pay for the benefits of the babyboomers, they invested the 2.5 trillion in the Federal government, and the feds spent the 2.5 trillion and more....
Now the the SS Funds needs those trillions back, where they are going to come from is... unclear at this moment......
Strictly speaking, yes these payments have no effect on the Federal deficit, you redeem a trillion in bonds, you have a trillion less in cash, but also a trillion less in liabilities....
But on a cash flow basis, where do you get the extra trillions you need to redeem SS Trust fund T-Bills in addition to the borrowing you might need for other government activities...? - This is where the crunch comes in....