WASHINGTON — 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.

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At least nine presidential candidates made pilgrimages to ethanol factories in Iowa 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.

For years, biofuel advocates have promoted the idea that backing ethanol is a litmus test for winning Iowa, site of the nation’s first presidential caucuses. That persists despite the 2016 Republican caucus victory of Ted Cruz, who criticized the U.S. biofuel mandate. Many former critics have reversed course after campaigning in Iowa, from John McCain and Marco Rubio to Bernie Sanders and Hillary Clinton.

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. A biofuel trade group estimates that the industry pumps about $5 billion into Iowa’s economy, or about 3% of the state’s GDP.

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 officer of Mighty Earth, a not-for-profit group that stationed activists in the state.

“Corn ethanol and soy biodiesel are even dirtier than dirty oil — and the path to building support for reform goes through Iowa,” Hurowitz said.

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 — some with visions of phasing out all liquid transportation fuels, whether made of corn or crude.

The National Wildlife Federation has armed presidential candidates with talking points crafted to help them prove that they support rural America and corn farmers without endorsing ethanol at the same time. Activists also conducted door-to-door petition drives, panel workshops and volunteer training to emphasize the issue.

“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.”

Fetcher used that strategy with 2020 hopeful Tulsi Gabbard after a “meet and greet” event in an Iowa City brewery in February.

Fetcher wound around tables and through a throng of voters to reach Gabbard, and told the Hawaii Democrat that, contrary to popular belief, Iowans prefer to fight climate change with wind power, solar energy and land conservation rather than corn-based ethanol.

“I am in complete alignment with what you are talking about,” Gabbard responded, her voice rising over the din in the crowded bar as she stressed that it’s important to ensure that farmers aren’t hurt.

Mighty Earth had other early successes, recording Warren saying she wants “better biofuels” and using video of Booker saying he supports ethanol to spur a follow-up conversation with his staff.

The chairman of Washington-based Mighty Earth is former California Rep. Henry Waxman. The group’s Iowa biofuel initiative was partially underwritten by Jerry Jung, a former chief executive of heavy-equipment dealer Michigan CAT who blames the U.S. biofuel mandate for declines in the Monarch butterfly population.

Opponents of the 14-year-old Renewable Fuel Standard, including oil companies and wildlife advocates, say the mandate that requires biofuel in gasoline and diesel drives farmers to plow prairie grasses and shrubs 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. California, for instance, approves ethanol under its aggressive low-carbon fuel standard requirements. And 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 the initial production of raw materials to its processing and eventual 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.”

Embracing ethanol has enduring political appeal as a way for candidates to show empathy for rural America — a powerful tool for urban Democrats trying to court Midwest voters. And some Democrats, such as Sen. Amy Klobuchar of Minnesota, 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.

The Trump administration gave the industry a major win by lifting restrictions on higher-ethanol E15 gasoline sales but also has been criticized for exempting dozens of oil refineries from biofuel quotas.

Polling shows that supporting ethanol still helps in Iowa. In a March survey conducted for Focus on Rural America, 84% of likely Iowa caucusgoers said they would be more inclined to back a candidate “who supports expanding production of renewable biofuels like ethanol and growing related jobs in rural communities.”

But ethanol isn’t a priority issue for most voters — it’s consistently outranked by other concerns such as health care, immigration and education. Biofuel didn’t even place in a list of nearly two dozen issues raised by likely Iowa voters as their most pressing concerns in an April Monmouth University poll.

Still, biofuel boosters are making appeals to Democratic candidates — and countering Mighty Earth’s campaign with one of their own. They are inviting candidates to tour manufacturing facilities and circulating briefing memos rebutting 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.

And ethanol producers have established a new group — called Biofuels Vision 2020 — to help candidates appreciate “the benefits of renewable fuels for the environment, energy security and Iowa’s rural economies.”

“It’s everything Democrats want to talk about in terms of engaging rural America. It’s climate, it’s jobs, it’s environment and it’s rural income,” said Monte Shaw, head of the Iowa Renewable Fuels Association. “If they’re willing to learn and have a science discussion, then we’re going to be fine. The facts are on our side.”