Herman Cain has benefited from his own strong debate appearances, his opponents’ ineptitude and a near-total lack of critical attention to his policies. The lack of scrutiny is not surprising, given the fixation on the better-known and -funded candidates. But rival candidates have also strategically ignored his views.

Mitt Romney is no doubt delighted to have Cain knock down Texas Gov. Rick Perry on immigration and political careerism. Perry doesn’t want to go after Cain for fear of letting on that he’s no longer on the same tier as Romney. Other candidates have been loath to attack the funniest, arguably most charming man in the race.

Cain can’t continue to duck questions. He won’t say if he agrees that Mormonism is a “cult,” but shouldn’t voters know his views on this? He acknowledged that he knows little about national security but why should voters trust yet another novice in such dangerous times. What’s remarkable is that his opponents and, more generally, the media haven’t pressed him on these topics or most anything else. Those days may come to an end quickly.

Perry needs to regain the base of support he has lost to Cain. Rick Santorum, who made a strong showing at the Values Voter Summit, needs all the Christian conservative votes in Iowa he can round up. Rep. Michele Bachmann (R-Minn.) likewise has to get back into the game.

The normal lines of attack against Cain — he’s never won an election, he doesn’t know anything about how Washington operates, etc. — would only endear him to the Tea Partyers. Perry can’t very well boast that he knows more about foreign policy than Cain does. Bachmann isn’t about to make the case that she knows how to get things done in D.C. So where is Cain vulnerable to criticism?

Sometimes the obvious line of criticism is the most compelling. Cain’s 9-9-9 plan is, his critics say, ill-conceived and dangerously naive. I spoke at length yesterday to Rich Lowrie, the “wealth management” consultant who has crafted Cain’s plan. It wasn’t always easy to get straight answers from him, but I learned a number of things that frankly aren’t going to play that well to voters.

His 9 percent sales tax has already been criticized by Santorum and conservative tax gurus. Lowrie says that the tax would have no exemptions. That means food, rent, everything would be subject to the tax. But doesn’t this sock it to the poor and middle class?

Lowrie insists it doesn’t because other “embedded” taxes (corporate taxes, payroll taxes) would be repealed. But most experts think the math here doesn’t work. Lowrie is unconcerned about the enforcement issues and the potential for off-the-books transactions. He insists that, with only a 9 percent rate, we could largely rely on “voluntary compliance.” He insists that politicians would find it hard to raise the sales tax because the rate would be clear to everyone. (Spoken like someone who’s never had a conversation with congressional Democrats.)

Cain’s plan is also vulnerable on the income tax side. After fencing with me for some time, Lowrie acknowledged that Cain didn’t care about progressivity. In devising the plan, Cain aimed for aimed for simplicity, transparency, and ”fairness” (in the “Webster definition” sense, he says, meaning that income is taxed the same for everyone).

Lowrie says it’s just “Washington thinking” to look at whether modest-income Americans will wind up shouldering much more of the tax burden. He repeatedly refused to say how much more of the tax burden would be borne by the poor and middle class than under the current system. But he implicitly acknowledged the problem by saying that the campaign would “fix this” with a new empowerment-zone plan that would be laid on top of the 9-9-9 plan and would presumably lower taxes in inner cities. But how fair is that to people living elsewhere? And aren’t we back to more complexity?

In sum, there is much about the plan that is unpalatable to voters, conservatives included. If Cain’s rivals want to stop his rise and get back in the game, they will need to start debating him on the merits of his ideas. For those who have no plans of their own (Perry), it may be tricky. (Santorum, who has already attacked the sales tax and has a tax plan to bolster domestic manufacturing and help middle-class voters, may have a somewhat easier time.) But whether on taxes or the debt, Cain’s competitors will need to make a decision: Give him a free pass, or begin to challenge the charismatic outsider.