Herman Cain, 999 Plan Come Under Fire At GOP Debate (VIDEO)

In the GOP debates, Herman Cain's most frequent topic of discussion is his 999 economic plan, which would make the corporate income tax rate 9 percent, the personal income tax rate 9 percent and the national sales tax rate 9 percent.

While he usually uses it as an applause line, Cain immediately faced tough questions about the plan from the rest of the GOP field during Tuesday night's CNN debate. The increased scrutiny from the other candidates likely reflects the fact that he has been rising in the polls and they now view him as a threat.

Rep. Michele Bachmann (R-Minn.) hit Cain for wanting to add a new tax -- a national sales tax -- and raised the specter of it eventually being raised to 90 percent.

"If we give Congress a 9 percent sales tax, how long will it take a liberal president and a liberal Congress to run that up to maybe 90 percent? Who knows?" she asked. "What I do know is that we also have to be concerned about the hidden tax of the value-added tax, because at every step and stage of production, you'd be taxing that item 9 percent on the profit. That's the worry."

When asked about criticisms that his plan would raise taxes on the middle class, Cain argued that it is "revenue-neutral" and "does not raise taxes on those that are making the least."

"The reason that our plan is being attacked so much is because lobbyists, accountants, politicians -- they don't want to throw out the current tax code and put in something that's simple and fair. They want to be able to continue to manipulate the American people with a 10 million-word mess," he said.

Former Pennsylvania Sen. Rick Santorum also went after Cain, pointing to an analysis by the nonpartisan Tax Policy Center that found 94 percent of Americans would see a tax hike under his plan and criticizing him with the "value-added tax" talking point.

Texas Gov. Rick Perry also criticized the plan, deploying tougher rhetoric than he's used in recent debates.

"Herman, I love you, brother, but you don't need to have a big analysis to figure this thing out," Perry said. "Go to New Hampshire where they don't have a sales tax and you're fixing to give them one. They're not interested in '9-9-9.' What they're interested in is flatter and fairer. At the end of the week, I'll be laying out a plan that clearly -- I'll bump plans with you, brother, and we'll see who has the best idea about how you get this country working again. And one of the ways right here in Nevada -- you got 8 plus percent. You want 9 cents on top of that and 9 cents on a new home or 9 percent on a new home, 9 percent on your Social Security, 9 percent more? I don't think so, Herman. It's not going to fly."

When former Massachusetts Governor Mitt Romney asked Cain whether his plan would replace the state sales tax, Cain said it wouldn't. Romney said he therefore agreed with Perry -- to which Cain replied that they were "mixing apples and oranges."