No, really, trying to cut SS would be the worst idea ever.
Not only would the AARP carve you up, your base walk out, your Democratic allies lose their minds and the press reframe your entire presidency as you vs. "the left", the Republicans would just get in "give us an inch" mode and start agitating for medicare cuts, too, since that's the one that might actually face sustainability problems.
Besides, would you really try selling the idea that people should be saving privately? NOW? Everybody is thanking their lucky stars that Bush didn't get his "reforms" through--boomers would be facing the prospect of Meow Mix as a dietary staple--and you want to dig up THAT festering corpse?
You're finding it harder and harder to avoid [i]nationalizing the banks[/i], I somehow don't think that's going to inspire people's confidence in private-sector retirement solutions!
(And if you just cut without ANY other option, say goodbye to your Democratic Congressional allies, because they'll either ignore you or lose their jobs. Or both.)
You're smarter than this, Mr. President. Don't let axe-grinding economists throw you off.