After his recent meteoric rise in the polls ”” a month ago he was registering in the low single digits; today he’s around 20 percent and challenging Mitt Romney ”” establishment politicos are starting to look into Herman Cain’s candidacy as legitimate, much as they did after Michele Bachmann won the Iowa straw poll and Rick Perry anounced his candidacy.

Even The Washington Post’s Chris Cillizza and Aaron Blake -”” bastions of the establishment media ”” have recently written a column entitled, “Why Herman Cain can win.”

Though Cain will likely turn out to be a flash-in-the-pan like Bachmann and Perry, it is worth taking a look at his singular economic policy prescription: his 9-9-9 plan.

The gist of his plan is this: replace all current taxes with a 9 percent business flat tax, 9 percent individual flat tax and 9 percent national sales tax. He would eventually replace it with just the Fair Tax, a national sales tax on all goods.

As a campaign tactic, it is brilliant. Cain needed something to distinguish himself from the other candidates, so he did what he does better than anybody else in the GOP: He came up with a catchy slogan. It does not matter what 9-9-9 means. All that matters is it’s catchy, easy to remember and includes enough conservative economic ideas, no matter how hastily it is thrown together, to win over the Republican base.

On a practical level, it stands almost zero chance of passing Congress. Most Democrats will reflexively vote against it, and even most Republicans, judging by the tepid responses to the plan by fellow Republican candidates for president, won’t sign off on it either.

How about judged on its merits? There, too, Cain’s plan falls short.

The arguments against a national sales tax are legion. It would be attacked from the left because a sales tax is an inherently regressive tax: It taxes a larger percentage of the poor’s wealth than the rich’s wealth.

But conservatives are not enamored with the so-called Fair Tax either. In order to raise enough revenue to replace the current tax code, the national sales tax rate would have to be 30 percent. That’s a lot of extra money to pay every time you purchase something.

Most conservatives would rather the existing tax code be simplified.

Cain’s 9-9-9 plan is all fluff, no substance. It’s a nice slogan filled with a grab bag of random conservative policies that, put together, make a bunch of mumbo-jumbo that would never pass.

This is really the problem of the entire Cain campaign. His other economic proposals are akin to “get the government out of our way” rather than showing how he will make that happen.

He’s even more vague on foreign policy. He has made a habit of saying that he will wait to say what he’d do on foreign affairs until he has all the information ”” presumably when he’s president. And recently, he said, “I’m ready for the ‘gotcha’ questions and they’re already starting to come. And when they ask me who is the president of Ubeki-beki-beki-beki-stan-stan I’m going to say, you know, I don’t know. Do you know? And then I’m going to say how’s that going to create one job?”

Sorry, but that doesn’t cut it. If Cain wants to be taken seriously as a candidate, he needs to start acting like a serious candidate.