But as Diane Lim Rogers of the fiscally hawkish Concord Coalition notes, Cain's plan is exactly what youdon't want to do in a weak economy. After all, if your a middle class worker, you'd pay 9 percent on all your income and 9 percent on your purchases which in lower and middle brackets constitute a significant percentage of your earnings. And even that doesn't count state sales tax.

"It's a cute little gimmick, but it's not an overall 9 percent effective rate," she said. "This kind of proposal would be bad for coming out of a recession. If you're trying to stimulate demand, you should encourage folks to buy goods and services. This proposal increases burdens on low income households and lessens demand."