I am not (at least not yet) a Cain supporter, I have concerns about all the candidates in the race, and I have not yet decided what I think about the 9-9-9 plan. However, I do think that the visibility of the tax could be a positive. Our national payroll tax deduction system had made our taxes invisible to a certain extent; unless we're self-employed and send quarterly checks to the feds, most of us never see the money we're paying in taxes, and many of us deliberately have extra money withheld each month so we get a tax refund.
Seeing every time we go to the grocery store that we're paying 9 percent federal sales tax could be a strong barrier to raising taxes beyond that percentage simply because we see how much we're paying every time we buy something.
I agree that seeing a tax every day is a way to keep it in people’s minds but that is exactly why it will hurt the candidate that proposes it. I’m not sure how many states have no state sales tax (very few I know) but any candidate that proposes adding a federal sales tax in those states will lose those states by double digits no matter what his position on other issues. And what about a state that has high sales tax already? You’re going to propose adding ANOTHER 9% on top of that? I don’t care how much you say you’re going to cut their income taxes that just isn’t going to fly.