Nationwide, 54 percent of Americans think the government should subsidize ethanol producers, according to Reuters/Ipsos polling. It is less popular among Republicans – only 46 percent support ethanol subsidies.

Neither Bush nor Walker, the Wisconsin governor, have weighed in specifically on the Renewable Fuel Standard before.

Walker opposed a similar state mandate as a candidate for governor in 2006. Bush has criticized ethanol subsidies more broadly on several occasions.

Both said the existing law will be needed for the next few years at least but that they hoped to do away with it eventually.