Republican presidential hopeful Rick Perry must be onto something with his flat tax. Liberals wasted no time on Tuesday shooting arrows at the Texas governor’s proposal, labeling it a giant-sized tax cut for millionaires and billionaires that’s paid for with higher taxes on the middle class.

Obama administration spokesman Ben LaBolt said the flat tax “would shift a greater share of taxes away from large corporations and the wealthiest onto the backs of the middle class.” Mr. Obama’s team just can’t get away from the class warfare theme.

The Democratic National Committee was also on the warpath. It sent out an email claiming to “fact check” the Perry flat tax, which would offer taxpayers the choice of filing under the current system or paying a 20% rate. The DNC cites former Obama administration economist Jared Bernstein, who says “not only would [the plan] shift the tax burden from the rich to middle-class and poorer taxpayers, but it would also shift the burden of tax preparation away from the rich.” Apparently, tax fairness means everyone has to share equally in the burden of tax complexity. Now, there’s an argument that only a tax accountant could love.

The Perry campaign has spent much of the past few days swatting down these “fairness” attacks. A Perry spokesman tells me that “nearly every income category comes out ahead financially under the flat tax. We were very careful to construct this in a way that protects the middle class from higher taxes.” Because the system is optional, no American has to pay a higher tax if they don’t want to. And like Herman Cain’s 9-9-9 plan, Mr. Perry’s proposal eliminates the corporate welfare in the code that allows companies like GE to pay no tax.

Now the pressure is on Mr. Romney to devise a bold tax plan of his own. He’s put out a detailed economic proposal with 57 planks and has called for tax rate cuts and other reforms, but he hasn’t put forward a comprehensive plan to overhaul a tax system that now extracts more than $300 billion a year in compliance and enforcement costs. Conservatives are challenging Mr. Romney to adopt a sweeping tax rewrite. He may have to do that if he wants to win the nomination, because right now the Republicans are a flat tax party.