Former President Donald Trump went after his main rival for the Republican presidential nomination Friday on an issue of out-sized importance to Iowa caucus voters: the federal ethanol mandate.

Trump told supporters in Council Bluffs, Iowa, that Florida Governor Ron DeSantis “totally despises Iowa ethanol.”

“He has been fighting for years to kill every single job supported by this very important industry,” Trump said. “If he had his way, the entire economy of Iowa would absolutely collapse.”

As a congressman in 2017, DeSantis co-sponsored the Renewable Fuel Standard Elimination Act, which would have done away with the Clean Air Act provisions requiring the use of ethanol in gasoline for cars and trucks.

That mandate is an important policy concession to Iowa, reinforced for decades by the state’s first-in-the-nation contest for party presidential nominations. Republicans have maintained Iowa as first in line — a policy Trump said he would continue as party leader — though Democrats have moved South Carolina to the top of their calendar.

Earlier: Ethanol Quota Cut Rattles US Sector Already Feeling a Squeeze

“Donald Trump’s attacks today are further evidence of his eroding support in Iowa. Iowans increasingly know that Governor DeSantis is the presidential candidate who shares their values,” said DeSantis spokesman Bryan Griffin. “As president, Ron DeSantis will be a champion for farmers and use every tool available to open new markets.”

National polls show Trump far ahead of his rivals, leading DeSantis by 32 points in the RealClearPolitics average. But states with early nomination contests are more competitive. The RealClearPolitics average finds Trump leading by 24 points in Iowa.

Trump stepped up his attacks on DeSantis over agriculture policy in his first rally in Iowa since March, when he visited the Mississippi River city of Davenport.

Council Bluffs, at Iowa’s far western edge and across the Missouri River from Omaha, Nebraska, is in the more agricultural region of the state. Iowa’s most important crop is corn, the most common source of ethanol. When he was in the White House, Trump visited the area to tout his administration’s actions on corn-based ethanol.

Trump’s Council Bluffs rally led with a 20-minute direct appeal to farmers before launching into his trademark freewheeling stump speech. He rolled out a “Farmers for Trump” coalition that included Iowa House Agriculture Committee Chairman Mike Sexton.

He also railed against electric vehicles, which he said would “totally kill Iowa ethanol.” Trump vowed to increase ethanol exports, slash regulations on farming and fund farm subsidies through increased tariffs.

(Updates with DeSantis campaign response, in sixth paragraph)