Paul Gigot and Dan Henninger discuss how well Herman Cain is weathering the scrutiny and preview tomorrow night's debate.

The two big questions now in the GOP presidential primary are whether anyone can emerge as the main challenger to Mitt Romney, and who that might be. First Tim Pawlenty faltered in that attempt, then Michele Bachmann and Rick Perry rose and fell, and now Herman Cain gets his moment to shine or become a fleeting memory.

In particular, we're going to find out if Mr. Cain's 9-9-9 tax reform plan can hold up under a sustained assault from the media and his competitors. Throwing out the tax code has enormous political appeal, but his plan's vulnerability is its new 9% national sales tax even as it retains the individual income tax. David Gregory gave Mr. Cain a grilling on NBC's "Meet the Press" on Sunday, and Mr. Cain's response was uneven. He did well to explain that the cost of goods would fall as the income and corporate tax rates fell, offsetting some or all of the sales tax increase. But he lacked an adequate reply to the dangers of a combined state and national sales tax that could reach upwards of 18% in some places, as well as the danger of giving the federal government a new revenue stream with a sales tax in addition to the income tax.

You can bet his competitors will drive home these points in Tuesday's debate. Newt Gingrich, who has also been rising in the polls, paid Mr. Cain the compliment of saying he could be the GOP nominee, but notice the back-handed delivery. "If Herman figures out how to do it all right, and if he can explain a 9% sales tax so people decide they want it, he has a good chance to be the nominee," Mr. Gingrich said on CNN's "State of the Union."

This is an important debate for Republicans to have, because you can be sure President Obama would mention that 9% sales tax every day through Election Day. The GOP's national sales tax supporters (notably the FAIR tax folks) have been looking for a champion for at least two decades, and in Mr. Cain they now have an articulate advocate who is staking his candidacy on the idea. If he can't make the sale, it's doubtful anyone can.
