Well, if they tried that, they probably would run into a real brick wall, because it would affect very large numbers of people. I'd expect them to try something like means-testing Social Security first, so they could try to sell it as more 'Soaking the rich,' but considering the unquenchable thirst of Democrats for other people's money, about anything is possible.
Their whole approach to taxation is divide-and-conquer; they are trying to hunt down numerically-small sources to screw over, and not from among their own key sacred cows, so that they won't pay a price in pissed-off voters (The actually-living part of their constituency that has a choice about voting Democrat, that is) come election time. Sometimes their greed outpaces their careful parsing of the electorate, which seems to be where they are headed lately.