First chinks appearing in the armor of Herman Cain's 9-9-9 plan?

Herman Cain is now entering his "flavor of the month" moment.

Since vaulting into the top tier of Republican presidential candidates this month, Mr. Cain has been trailed by a single underlying question: How long can he last? In a primary season that has seen challengers to Mitt Romney rotated and then cooked like a rotisserie chicken, is Cain simply the next in line or the real deal?

The upcoming GOP debate Tuesday will be an important test – the first since Cain topped the NBC/Wall Street Journal poll, and the first time that he's likely to be tested on topics beyond the economy. Yet mounting attacks on his economic platform – specifically, his signature "9-9-9" tax plan – from liberals and conservatives perhaps carry the greatest danger.

Sunday gave at least a glimpse of how he might respond. For the most part, Cain's appearance on "Meet the Press" showed a deftness and a depth of understanding that has so far eluded the previous would-be challenger to Mr. Romney, Rick Perry.

But the exchange appeared to show three potential chinks in the armor of 9-9-9 that could resurface.

1. The poor

Nonpartisan tax analysts are pretty much agreed on one thing about 9-9-9: It would be a terrible thing for the poor. The reason is that many of the poor pay no income taxes, nor Social Security or Medicare taxes. So wiping away the entire tax structure and replacing it with a 9 percent income tax for everyone actually forces them to pay more. In addition, the poor – like everyone else – will have to pay 9-9-9's new 9 percent federal sales tax, but are the least able to afford it.

So it's a double whammy.

David Gregory, host of "Meet the Press," tried to pin Cain down on this: "For those 30 million Americans who don't pay income tax, including 16 million elderly Americans, you concede they would, in fact, pay more?"

Cain interrupted: "Not the elderly. That's two different groups." He said the elderly would be helped by eliminating taxes on Social Security income, capital gains, and investment dividends. But the other group of Americans who pay no income tax – the poor? Cain left that untouched.

Cain does have a rebuttal to cries that 9-9-9 is unfair to the middle class: "Some people will pay more, but most people will pay less," he said.

It's unclear whether that is true, and some economists are dubious. But it's a defensible line, particularly in a Republican debate. Cain insists that simplifying the tax code would have knock-on benefits – such as lower prices and more jobs – that make it a net win for the middle class. That's straight from the tea party playbook.

It will be harder for him to make that same argument about the poor. But what Republican is going to challenge him on that?

2. Sales tax rates

If 9-9-9 were to become law, a new 9 percent federal sales tax would be added to state and local sales taxes nationwide. In some states, like Tennessee, this could lead to an average sales tax rate statewide of more than 18 percent. Hardly a selling point.

On Sunday, Gregory tried to pin Cain down on that, too. Cain's response was to try to take state and local sales taxes out of the conversation.

"Don't combine it with state taxes," he said. "This doesn't address state taxes. If you add them together, yes, you would get that number. [But] this is a replacement structure. These are replacement taxes. They're not on top of anything."

Gregory persisted, trying to force Cain to acknowledge the high cumulative rates. But Cain refused to budge.

Under 9-9-9, he repeated, "Your state taxes are the same. Your federal taxes, in most cases, are going to go down. [Combining the two is] muddying the water."

We understand Cain's argument. He's looking at it holistically. Overall, he contends, 9-9-9 is a good thing, even if those sales-tax numbers are a little ugly.

But they are ugly, and some Republican could make that stick.

3. A new tax

Within Republican circles, Cain's biggest challenge in defending 9-9-9 might be the idea of a national sales tax in the first place. Tea party gospel calls for starving the federal government of money by cutting taxes, then insisting on a balanced budget – forcing the government to shrink.

Adding a new tax – a completely new source of revenue for the federal government – is like giving Dr. Frankenstein jumper cables.

Cain insists the rate will stay at 9 percent. Antitax advocates say such promises have been made – and broken – before.

Cain's response is that 9-9-9 is tinker-proof. Its security is its simplicity: Nine percent federal tax rates for sales, income, and corporations.

"What 9-9-9 does, it makes it very visible, such that the American people can hold the feet of Congress to the fire," Cain said.

For conservative ideologues with long memories, however, it would be a Faustian bargain.