Byron York

Herman Cain's supporters know their part by heart. On the campaign trail, at the point in Cain's stump speech when he begins to discuss his plan for economic growth, they're always ready to join the chorus: "Nine! Nine! Nine!"

They're referring, of course, to the Republican presidential candidate's proposal to throw out today's tax structure and replace it with a 9 percent income tax, a 9 percent business tax and a 9 percent national sales tax. Cain would eliminate capital gains taxes, the payroll tax and the estate tax.

For Cain, a Georgia businessman, 9-9-9 is a perfect platform. It's specific, but it doesn't bury people in details, like Mitt Romney's 59-point, 160-page plan. And it's not a vague promise like Rick Perry's look-what-I-did-in-Texas position. To a lot of voters, 9-9-9 is an enormously appealing proposal that is easy to grasp.

It's audacious, too.

"I had a kind of pivotal moment in this," says Rich Lowrie, the head of an investment firm in Cleveland who serves as Cain's top economic adviser. "I was with Mr. Cain and I asked him, 'How bold do you want to be?' and he leaned toward me with his big, booming voice and said, 'BOLD.'"

So bold it was. But is 9-9-9, for all its boldness, a good idea?

I talked with a number of conservative economic policy experts who don't want to take sides in the campaign and thus asked to remain anonymous. They found some important things to like in 9-9-9. They favor its low rates, and they like its elimination of various types of double taxation. Most agree it would stimulate growth and create jobs, at least in the short run.

But they have two serious objections. The first is that 9-9-9 might not raise enough money to fund the government even if it creates growth and federal spending is reduced. Over the years, the government has taken in tax revenues equal to about 18 percent of gross domestic product. "I'd be surprised if 9-9-9 raises as much money as current policy," says one expert. "I'd be really surprised if it raises 18 percent of GDP."

Cain's advisers have put together a detailed analysis, or score, to argue that 9-9-9 would be "revenue neutral," that is, it would raise the same amount as today's system. "We used 2008 as our baseline, and not accounting for any growth effect, it would have generated to the penny the same revenue," says Lowrie. So far, though, the numbers have not been crunched by many experts outside the campaign.

Byron York

Byron York, chief political correspondent for The Washington Examiner