Sen. Ted Cruz (R-Tex.) campaigns at Penny's Diner in Missouri Valley, Iowa. (Nati Harnik/Associated Press)

WE CAN think of dozens of good reasons why Sen. Ted Cruz (R-Tex.) should not win the Iowa caucuses. Unfortunately, Mr. Cruz’s opponents seem to care only about bad ones.

One such is Mr. Cruz’s support for ending Washington’s ethanol mandate, which requires that increasing amounts of biofuel be blended into the national gasoline supply. Mr. Cruz wants to phase out the mandate. Iowa agricultural interests — including a pressure group led by Republican Gov. Terry Branstad’s son — have slammed the senator for supposedly putting Big Oil above Big Corn. Their mobilization includes anti-Cruz commercials, pamphlets and a pro-ethanol trailer following Mr. Cruz from campaign stop to campaign stop. Other candidates, such as Sen. Marco Rubio (R-Fla.), are rushing to claim any Iowa voters who turn away from Mr. Cruz, playing up support from the agriculture lobby.

In fact, among the only positive things about a Cruz win in Iowa would be the damage such a result would do to the ethanol interests. Since Congress handed them their ethanol mandate a decade ago, every winner of the Iowa caucuses has supported it. Iowa’s favored status as the first-in-the-nation caucus state has made repealing the mandate all but impossible.

But there is no doubt it should be repealed. Blending more and more ethanol into gasoline will require spending money on infrastructure that is not yet in place and selling more fuel that older and more specialized engines cannot take. It will also raise food prices, according to a 2014 Congressional Budget Office analysis.

And the supposed benefits? The country’s recent uptick in oil production eliminates the already weak argument for ethanol’s energy security benefits. Another major purported benefit is a claimed reduction in transportation sector greenhouse emissions. But the CBO found that “replacing gasoline with corn ethanol has only limited potential for reducing emissions (and some studies indicate that it could increase emissions).”

A more recent paper in the journal Science found that the models predicting emissions reductions come to that conclusion by assuming that less corn would be used for feeding people and animals. In other words, people will eat less and produce less carbon dioxide in the process. “Any reduction in global food consumption is likely to disproportionately affect some groups of the poor because they can less afford higher prices,” the paper noted. True, farmers can try to grow more food on the land that is already in use. But they already have to do that to feed a rising global population. Putting further strain on the global food supply is a high price to pay for any marginal benefits to greenhouse emissions.

The ethanol mandate serves little purpose beyond subsidizing the farming business, which already gets massive federal subsidies. Mr. Cruz is right to favor phasing it out, and his major opponents are nothing but ethanol opportunists.