Herman Cain really doesn’t know when to quit. The former pizza chief and failed presidential candidate announced on Sean Hannity’s FOX News program last night that he is launching a bus tour that will kick off “Cain’s Solution Revolution.” And the first item of business is to tout his seriously suspect “9-9-9” plan.

Cain told Hannity that one of the goals is to get members of Congress to sign on to “9-9-9” before Election Day. He said that legislation is being drawn up. The problem he’s going to have is finding anyone willing to put their name on a plan that folks on the right, such as Grover Norquist, the left and in the middle have slammed as flawed. Even The Post Fact Checker gave “9-9-9” three Pinocchios because “the plan he touts as a big tax cut would actually increase taxes on most Americans.”

And you know there’s a problem when Cain can’t even explain it, as happened on “Meet the Press” last October. When pressed by moderator David Gregory on criticism that the middle class and poor would pay more under “9-9-9,” Cain argued that that wasn’t the case because the “invisible taxes that are built into everything we buy . . . are going to go away.” And then famously said this when asked again who would pay more.

Who will pay more? The people who spend more money on new goods. The sales tax only applies to people who buy new goods, not used goods. That’s a big difference that doesn’t come out.

Again, I ask, if you buy a used loaf of bread or other used necessities, you will avoid the national sales tax? None of this makes sense. None of it. But what does make sense is Cain’s insatiable appetite for attention. We’ll soon see whether he gets any real takers to sign on to “9-9-9.” Maybe Mitt Romney will “see the light” in yet another craven bid to ingratiate himself to the conservative base. Or maybe not. Somehow, I don’t think Cain really cares all that much.