Think about what the dems can do with the Fair Tax once they regain control on the Hill. It's a VAT just waiting to happen.
To overturn a tax law of the land that will be very popular with the people - as the FairTax will be - it would be very difficult to make it into a VAT.
Remember, there are plenty of bad experiances with VATs to point to and there would be a lot of opposition ... even within the Decocratic party.