I don’t know if I would go quite so far as to say that a “grand bargain” including Social Security cuts would destroy the Democratic coalition. But it’s certainly true that it wouldn’t attract anybody to the Democratic coalition — since Pain Caucus pundits don’t actually represent any meaningful constituency — and it would give a lot of people less reason to vote for the Democrats. The only question is how big the downside would be.
And, of course, not only are the politics terrible, but the whole idea of “grand bargains” is senseless even on their own dubious terms. Even if you succeed in reducing deficit projections, it will all go to upper-class tax cuts the second Republicans control the White House and House of Representatives simultaneously.