I agree with Deadman that "the idea of social security is to provide the elderly with a baseline level of resources so they don't become impoverished".  I think that a major reason that Social Security reform is the "3rd rail" of politics is that so many people disagree with us.  If you listen to the AARP crowd, they are adamant that this is not the case. 

They think that that makes it sound like a kind of welfare program for the elderly, and given that many of them live on their Social Security (and that they often have Reaganesque attitudes about welfare) they get insulted.  I've got no desire to disparage the elderly.  I understand that there are a lot of factors at play.  There may be a cultural attitude of  fierce self-reliance which is already eroded by the realities of retirement and the loss of independence which accompanies decling health.  If you make an assertion which may be perceived as a false accusation that they're living off of what amounts to 'government handouts', you will spark resentment.

They point to the fact that they paid into it when they were working and may believe that they're just
getting back what they're owed.  You can point out that the benefits paid out to the average individual far exceeds the value of what was paid (even if invested).  You can try to disabuse them of the notion that Social Security works like pensions in the private sector.  You can point out that there is no pension fund which holds real assets, but a trust fund which  makes claims on the treasury that end up getting paid with taxes.  You can talk about unfunded liabilities and having to force a smaller generation of workers to fund the larger generations's retirement.  The problem is that you can only do this  if you're willing to alienate them- which is something no one facing reelection will do.

(AARP is a dragon killer.)