Supply-side economics is supposed to be a cheery creed, and its devotees have good reason to be happy. Keeping the top federal tax rate low has traditionally been their key policy objective. Today, that rate is 35 percent: It has been lower than that for only five of the past 80 years.

Yet in this campaign, the keepers of the supply-side faith are grim. So far the two Republican candidates who have departed the most from supply-side doctrine have done the best.

All the Republican candidates are calling for more supply-side policies.

Former Massachusetts Governor Mitt Romney wants to abolish the estate tax. Former Senator Rick Santorum wants to cut the top tax rate to 28 percent. But Wall Street Journal columnist Kimberley Strassel writes that both Romney and Santorum (who finished first and second in the Iowa caucuses, respectively) are “following Democrats down the class-warfare rabbit hole.”

Neither candidate is talking about raising taxes on the rich. Romney’s sin is to have proposed reducing taxes on investments only for people making less than $200,000 a year. Santorum’s is to have called for tax cuts for manufacturers and for increasing the value of the tax exemption for children.

Blinkered Vision

Some of these objections have merit, and some don’t. (All of them are overwrought.) There’s no good reason for the government to give special breaks to manufacturers. Romney’s proposal seems ineffective either as a way of offering tax relief to the middle class or of promoting investment. Santorum’s increase in tax benefits for children, on the other hand, is a modest but praiseworthy step toward reducing the federal government’s overtaxation of parents. Strassel’s treatment of parents as a special interest with an eccentric hobby is a reflection of the blinkered vision to which too many supply-siders are prone.

Purists have long insisted that their supply-side ideas represent both sound economics and smart politics. But it doesn’t seem to be working out that way this season.

Last June, former Minnesota Governor Tim Pawlenty tried to position himself as the boldest tax-cutter in the race, proposing a reform that brought the top rate down to 25 percent. He met the supply-side demand for optimism by promising his plan would yield 5 percent growth for 10 years. Within two months, he was out of the race.

In August, former Utah Governor Jon Huntsman came out with his own plan to eliminate almost every tax break -- including the child tax credit, which the Wall Street Journal has always loathed -- and reduce the top tax rate to 23 percent. The Journal called his plan “impressive” and said it “may start getting him more attention.” It didn’t.

Newt Gingrich and Rick Perry proposed allowing people to pay taxes either under the current tax code or under a new one with lower rates and fewer deductions. The Journal described Perry’s plan as “a chance to rejuvenate his candidacy.” Perry kept sinking. In December, Strassel reported that Gingrich had started to emphasize his bold tax-cutting plan as a way to return to front-runner status. She worried that this tactic came too late to save him. Actually, it doesn’t seem to have even arrested his slide.

Herman Cain is the only example of a candidate who moved ahead in the race based on his advocacy of a bold supply-side tax reform. But it was the seeming simplicity of his “9-9-9” plan that drew fans more than its pro-growth elements, which may be why the Journal kept its distance. And Cain quickly ran into trouble answering questions about the plan -- especially explaining whether it would increase taxes on the poor -- and had to change it on the fly.

Diminishing Dividends

It’s still a supply-side party. And in none of these cases could it be fairly argued that the candidate’s tax proposals caused him to lose altitude. But supply-side boldness clearly isn’t paying the political dividends its advocates say it should.

The reasons are not hard to guess. The deficit may have finally gotten so large, and our long-term fiscal outlook so bleak, that even Republican voters are less enthusiastic about massive tax cuts that come without matching spending reductions (as in the Santorum, Gingrich, Perry and Pawlenty plans). And the proportion of voters who pay the income tax has fallen a lot since the start of the tax revolt in the late 1970s, so tax plans centered on cutting income-tax rates have less appeal.

I’m still a fan of supply-side economics in general and the Wall Street Journal editorial page in particular. Taxes do reduce the incentive to work, save and invest. We should strive to keep them low. They should be designed to distort economic decisions as little as possible. But these truths, like any truths, can be pushed too far.

The progress of the Republican race suggests that supply-side politics, in its present form, is hitting its limit.

(Ramesh Ponnuru is a Bloomberg View columnist and a senior editor at National Review. The opinions expressed are his own.)

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To contact the writer of this article: Ramesh Ponnuru at rponnuru@bloomberg.net.

To contact the editor responsible for this article: Timothy Lavin at tlavin1@bloomberg.net.