The 9-9-9 plan can't be ignored, nor brushed aside, because it taps into powerful sentiments among Republican voters. The Cain campaign video promoting it is chock-full of red-meat bashing of the Internal Revenue Service; in a year when anti- government feelings are running at an unprecedented intensity, there can be no underestimating the power of that message for the party's primary voters.

Lower Incomes

Cain's plan also responds to Republican calls to lower corporate taxes by proposing the steepest reduction of those rates in history. It taps into the Republican belief that too many lower-income workers "avoid" paying income taxes, and would increase the burden on tens of millions of low-wage families.

It scratches the Republican itch for lower taxes for the wealthiest by cutting the federal income-tax rate on the highest earners to a stunningly low level: A person earning $1 million a year who now pays, on average, about $260,000 in federal income tax (after deductions and exemptions), would pay just $90,000.

As a result, the Republican candidates -- especially the front-runner Mitt Romney and the conservative establishment candidate Rick Perry -- are at a decisive fork in the road: Embrace 9-9-9, or oppose it. Neither is a very good option.

To oppose the plan in the Republican primaries is to take a position that will be seen by the party's voters as a defense of the status quo, in a year when the energy and enthusiasm are with the grass-roots, insurgent wing.

Emotional Appeal

Romney (to a great extent) and Perry (less so) are already vulnerable among Republican voters as "insider" candidates; standing against the most audacious "change" plan in the Republican universe will only reinforce that perception. Trying to pick it apart with facts and figures (a strategy foreshadowed by some campaigns' comments) will be likewise unavailing, given the emotional and evocative appeal that 9-9-9 has for the party's base.

But conversely, if Romney or Perry embraces the plan to gain favor with primary voters, he would court disaster in the general election. For tens of millions of Americans making between $40,000 and $100,000 a year, and who spend virtually every dollar they earn on basic goods that would be subject to a new 9 percent federal sales tax under Cain's plan, 9-9-9 is a massive tax increase. A typical family of four making $70,000, paying about $7,000 today in payroll and income taxes, would owe more than $12,000 in federal income and sales taxes under Cain's blueprint.

General Election

Right now, Republican primary voters who are moved by rabidly anti-IRS, anti-government zeal are prepared to embrace the 9-9-9 plan regardless of its effect on their personal pocket books; in the general election, where pragmatic, non-ideological voters are the swing bloc, the 9-9-9 proposal's impact on middle-class voters would be fatal.