Sorry, but the government can raise ANY tax.
The same thing that would stop the government from raising any tax is the taxpayer.
I would argue there are actually more limits under 999:
- is transparent, so everyone will see what Congress is trying to do if they try to raise rates.
- because everyone is paying, there is no class warfare. If they try to raise the rates, there will be public outcry of raising taxes on the poor.
- Cain will ask Congress to put in 2/3 majority rules to keep from changing rates - if they will is another question.
Oh, but there would be if the 'Rats were back in: they'd immediately start monkeying around with the simple concept to make it complicated in the name of their twisted notion of "fairness". Let's see, were to start: they'd exclude food and medicine from the sales tax, raise rates across the board with an emphasis on the corporate tax and reintroduce a bracketed income tax.
The only ways to make Cain's proposal entirely salutary involve Constitutional amendments: repealing the 16th and/or a Constitutionally required supermajority requirement for tax rate increases. I still like Cain, probably better than anyone else in the race (though Gingrich and Santorum would be fine, too, and I could take Perry or Bachmann), but as proposed 9-9-9 isn't entirely something I could get behind.