Shockingly, Republicans are smart enough to understand that people like Social Security.
It's one thing to vote for a "grand bargain" because you think it's on balance a decent compromise, it's quite another to put yourself on record as supporting cutting an extremely popular program when there isn't even anything to vote on.
The great mystery to me is why Dem candidates aren't smart enough to promise to give more stuff to the elderly. The elderly vote. Hell, I'd be happy for candidates to just lie about it, if only to prove that they have half a clue about what might appeal to voters.