I'm always befuddled by this argument. How much harder would it be for Congress to raise a sales tax than jack the income tax back up to 90%? What's to stop them from raising the corporate tax rates right now to 90%? No one can stop a future congress from raising taxes except the voters. That has never changed. Right now there are several hidden taxes that Congress can raise without most people noticing. They would notice a sales tax increase, Conservative or Liberal. Cain and Gingrich want to take away the class warfare card and make everyone part of the system. I'm not against that.
We just don’t like giving Congress a whole new field to plow looking to get taxe money from the American people. Plus sales taxes are the province of states and counties, not the Feds. They get you on income and capital gains.