– Environmentalists are taking their case that corn-based ethanol is bad for the planet to the state that makes more of it than any other: Iowa.

They are bird-dogging presidential candidates such as Elizabeth Warren and Cory Booker at rallies and town halls, trying to dissuade them from making politically convenient pro-ethanol pledges to get votes in corn country. Their message: biofuels are driving environmental harms, from disappearing wetlands to algae blooms in the Gulf of Mexico.

With Democratic 2020 candidates flocking to Iowa, biofuel foes are challenging conventional wisdom that ethanol support is untouchable in Iowa. So far, their efforts aren't working.

At least nine presidential candidates made pilgrimages to ethanol factories in Iowa so far this year, including President Donald Trump, who visited the Southwest Iowa Renewable Energy facility in Council Bluffs on June 11.

Former Democratic congressman Beto O'Rourke toured the Big River Renewables LLC ethanol plant in West Burlington two days after announcing his bid. New York Mayor Bill de Blasio praised biofuels as good for the planet at a Poet biorefining plant in Gowrie in May. On Tuesday, Rep. Tim Ryan stopped by the Golden Grain Energy LLC ethanol facility in Mason City.

Showing love for ethanol is part of the political script in Iowa. "You've got to get a picture in a cornfield, if you can find one, and you have to have the picture with an ethanol plant and then you've got to have the picture with the corn dog," said Chad Hart, an agricultural economist at Iowa State University in Ames.

Iowa produced 2.5 billion bushels of corn last year — with 62% of that going to make ethanol. And the state leads the nation in producing biodiesel, a renewable fuel typically made from soybeans.

Biofuel foes view Iowa as the front lines of their fight. Campaign promises made there have a direct link to pro-biofuel policies in Washington, said Glenn Hurowitz, chief executive of Mighty Earth, a nonprofit group that stationed activists in the state.

Biofuel critics saw an opening this year. The Democratic field is crowded with coastal lawmakers who don't have entrenched positions supporting renewable fuel. And many of the Democratic contenders are competing to outline bold plans for combating climate change.

"A lot of the candidates come in here assuming they have to talk about how great ethanol is because they think that's what Iowans want to hear," said Anya Fetcher, an activist who led Mighty Earth's campaign in Iowa City. "It's our chance to shape the narrative and let them know early on that it is important to talk about real climate solutions."

Opponents, including oil companies and wildlife advocates, say the Renewable Fuel Standard that requires biofuel in gasoline and diesel drives farmers to plow prairie grasses to grow corn and soybeans. And they argue that greenhouse gas emissions associated with corn-based ethanol are higher than anticipated when land conversions are factored in.

Federal and state agencies generally treat ethanol as a climate-friendly alternative to petroleum-based gasoline. A U.S. Department of Agriculture study published in April credits corn-based ethanol with producing at least 39% fewer greenhouse gas emissions than gasoline over its entire life cycle, from production of raw materials to combustion in vehicles.

Biofuel advocates dispute claims of rampant land changes they say are based on flawed research. In the middle of an agricultural crisis, "farmers are trying to hold on to the land they have," said Brooke Coleman, executive director of the Advanced Biofuels Business Council. "They're not clearing more land. They're trying to survive."

Some Democrats, such as Sen. Amy Klobuchar, see the issue giving them an opening against Trump, who promised to "protect" ethanol on the campaign trail but has been more equivocal in the White House.

But ethanol isn't a priority issue for most voters — it's consistently outranked by others such as health care, immigration and education.

Still, biofuel boosters are making appeals to Democratic candidates. They are inviting candidates to tour manufacturing facilities and circulating briefing memos refuting Mighty Earth's arguments. One of the biggest U.S. ethanol producers, Green Plains Inc., plans to "engage with candidates on pro-ethanol policy," Chief Executive Todd Becker told analysts on an earnings call.