“So why, if these two tax systems are really the same, are conservatives attracted to the flat tax and repelled by the VAT? It is because the flat tax is usually proposed as a substitute for our current tax system, whereas the VAT is often suggested as an addition to it. The bottom line, from both political perspectives, is that a VAT is neither blessed nor evil. It is a tool. We can use it to advance a larger government, a more efficient tax system or some combination of the two. That will be the key issue in the coming debate.”
http://www.nytimes.com/2010/05/02/business/02view.html?ref=business