Donald J. Trump attacked Gov. Ron DeSantis of Florida on Friday over his support for farmers, saying his chief rival for the Republican presidential nomination would be “a catastrophe” for the country’s agriculture industry.

Mr. Trump claimed at a rally in Iowa that Mr. DeSantis would outsource American farming jobs overseas and oppose the federal mandate for ethanol, a fuel made from corn and other crops. Support for ethanol, which Iowa is a national leader in producing, is a quadrennial issue in presidential elections in this early voting state.

In 2017, Mr. DeSantis supported legislation that would end the renewable fuel standard, a nearly two-decade-old standard that requires refiners to blend biofuel into gasoline nationwide. The policy is opposed by some conservatives, who see the mandate as onerous government regulation.

Speaking to hundreds of supporters in Council Bluffs, Iowa, on Friday, Mr. Trump rattled off his record of delivering on priorities for conservative farmers, including raising the exemption limit on the estate tax and replacing the North American Free Trade Agreement. Then, he eagerly highlighted what he claimed was his rival’s history of opposing an issue that carries outsize political weight in Iowa.

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

A spokesman for Mr. DeSantis did not respond to a request for comment.

The event marked Mr. Trump’s first large event in the state in nearly four months, after a rally scheduled for May was canceled by the campaign, which cited possible severe weather.

Held in a convention hall near the Nebraska border, the rally was packed with voters from the neighboring — and non-early voting — state.

“I hope Nebraska is represented here,” Mr. Trump said as the crowd exploded in cheers. “That’s a big contingent.”