Republican presidential candidates ramped up criticism of Herman Cain's tax plan that includes a 9 percent national sales tax.
Rep. Michele Bachmann of Minnesota said during Tuesday's GOP debate that there's no reason why a future Congress and president couldn't raise that to 90 percent.
Former Sen. Rick Santorum of Pennsylvania says 84 percent of Americans would see a tax increase under Cain's plan. He cited an independent analysis from the Tax Policy Center.
Former Massachusetts Gov. Mitt Romney says he likes Cain's boldness but his plan would raise taxes on the middle class.
Cain defended his plan and says it is a jobs plan that wouldn't add to the national debt. He says the plan faces criticism because it challenges the status quo and his rivals don't understand it.