No doubt Democrats hoped that Republicans would "compromise" and renege on the no-taxes pledge that is a key element of their "brand" -- thereby enraging their base and all but ensuring the emergence of a competing third party that would thus guarantee Dem electoral dominance. Fortunately, that didn't happen.
One final note: Through such veto threats, the President may hope and believe that he will somehow gain political credit for being a "deficit hawk." But it's worth considering that this spectacular failure of politicians and Big Government may well, at the margins, do more to hurt the image of the party (and President) who is constantly insisting that we should put our trust in Big Government -- not the party that believes in the primacy of the private sector.
No one serious could have truly expected for the Super Committee to come through with a serious compromise package that could win bipartisan support.