I think it was Ron Paul who was pushing the FairTax a while back. I can see the Dems embracing that at some point, with a new name of course. Because that's the path to a VAT. They can't pay for all this by simply increasing taxes alone, they'll have to widen the tax base (and increase taxes).
It's going to be brutal, and we'll start seeing them chip away at it soon. The "Wall Street tax" - which, by the way, I think FairTax needed to tax financial transactions as well - has been talked about over in Europe, too, and I think it gains steam because it both sounds like soaking the rich, while subtly expanding into the middle class (but only those with enough savings to buy stocks). And climate change excise taxes on energy and property because, again, it's a form of consumption and wealth tax that will hit people with larger homes.
Huge tax increases on the middle class seem inevitable. Only question is how long before it happens. I'll peg it not as the next Dem POTUS, but the next one after that (so maybe 15-20 years).