It's either flat or it isn't. Sales taxes vary widely by state. In Alabama (the last I checked), everything is subject to the sales tax. In Colorado, prepared food (e.g. restaurant meals) are taxed but ordinary groceries and prescription drugs aren't. In Oregon, there is no sales tax but other taxes are reportedly pretty high. As with anything, the devils in any national sales tax proposal are in the details. What would be subject to the tax and what would be exempt? Would it cover things like houses and rent? There would be a lot of pressure to exempt many of life's "essentials", but that means the tax rate on everything remaining would have to be increased to make up for the revenue shortfalls.

My opposition to the sales tax is primarily the 16th Amendment. If that isn't repealed, then it's inevitable that before long, we'd end up with both a national sales tax and an income tax. I see no chance that the 16th Amendment will get repealed, so I have a hard time supporting a national sales tax.

A flat tax, perhaps with generous personal exemptions and NO other deductions, could be both revenue neutral and would save over $100 billion a year in compliance costs. Tax accountants, tax lawyers, and tax software preparers would be the big losers in such a deal so you can expect them to oppose it with every fiber of their being. Everyone else would come out way ahead.