DES MOINES, Iowa - Talk about juice.

Bruce Rastetter, a multimillionaire agribusiness baron and the largest GOP donor in Iowa, got almost all of the potential 2016 Republican presidential candidates - at least 11 of them - to agree to come to the Iowa State Fairgrounds on Saturday for a forum on agriculture issues.

They were to sit down on stage for one-on-one, 20-minute interviews with Rastetter, discussing their positions on labeling for genetically modified food, proposed federal regulation of groundwater, foreign trade agreements - and, perhaps above all, the future of Iowa's important corn-based ethanol industry.

"We're looking to have a substantive discussion on an important segment of the economy that feeds the nation and the world," Rastetter said. "It's much broader than ethanol."

Yet there is a major bipartisan push in Iowa, led by the industry and Republican Gov. Terry Branstad, to make saving the federal renewable fuel standard a cause in the state's first-in-the-nation presidential caucuses. Some in Congress want to get rid of the rule, which requires blending ethanol and other biofuels into the nation's gasoline supply.

"This is the government using corporate welfare to shower money on a favored industry," Sen. Pat Toomey (R., Pa.) said Feb. 26 as he introduced legislation with Sen. Dianne Feinstein (D., Calif.) to eliminate the renewable-fuel requirement. The Obama administration wants to reduce the requirement but not eliminate it. Direct ethanol subsidies expired in 2011.

As domestic oil and gas production have increased in recent years, political support for ethanol has declined. Production of the fuel has helped create a glut of corn, with prices sliding from more than $8 a bushel in 2012 to less than $4 now.

Earlier this year, tractor maker John Deere furloughed about 800 workers in Iowa, and some worry about other ripple effects as farmers spend less.

"There's been a softening in the agricultural economy and a tremendous amount of apprehension about the renewable fuels standard," said Steffen Schmidt, professor of political science at Iowa State University. "You've got billions in investment in ethanol plants at risk and farmers losing their shirt on corn."

The fuels question sets up another test for candidates already navigating between the pro-business and tea-party wings of the GOP on immigration, the Common Core education standards, and whether to renew the Export-Import Bank. Many conservatives are loath to interfere in the free market.

"It will be interesting to see how the candidates walk the fine line between alienating the small-government Republicans and the farm interests," Schmidt said.

Some potential Republican candidates who have opposed the renewable-fuel standard, such as Florida Sen. Marco Rubio and Louisiana Gov. Bobby Jindal, will not be at the Iowa Ag Summit. Sen. Rand Paul of Kentucky, a libertarian-leaning contender, also is not attending.

But Sen. Ted Cruz of Texas, a cosponsor of Toomey's anti-ethanol bill, is scheduled to appear.

Two backers of the ethanol standard - former Pennsylvania Sen. Rick Santorum and former Arkansas Gov. Mike Huckabee - were to attend. They are the last two winners of the Iowa caucuses. Mitt Romney, who finished second in the state in 2012, held off backing the fuels standard until just before the voting, when he endorsed it.

Former Florida Gov. Jeb Bush will be making his first visit to Iowa in three years for the summit and also will meet with potential voters in Cedar Rapids. Gov. Christie also was to be there, though he had no public events planned. Neither potential candidate apparently has taken a position on the fuels standard.

Also scheduled to participate in the summit were Wisconsin Gov. Scott Walker, South Carolina Sen. Lindsey Graham, real estate mogul Donald Trump, former Virginia Gov. Jim Gilmore, former Texas Gov. Rick Perry, and former New York Gov. George Pataki.

Democrats mentioned as possible presidential contenders, including Hillary Rodham Clinton and Vice President Biden, were invited to the event but declined.

The Iowa Ag Summit will put a spotlight on Rastetter, 58, called by friend and foe alike an Iowa kingmaker. He's donated about a half-million dollars in federal races and $1.1 million in Iowa races since 2003, according to campaign-finance disclosures. Those public numbers don't count any contributions to super PACs that don't have to identify their backers.

Rastetter said that he's no power broker, just a concerned citizen, and that the candidates are drawn by the chance to address an important audience in a key state, not by him.

"Compared to the money they can get from other people, I pale in comparison," Rastetter said. "I don't have that kind of wealth. I feel fortunate to be able to help in Iowa and, in a small part, nationally."

He made his fortune in pork production and ethanol, and now his Summit Group is involved in farm real estate and private equity investment. Rastetter also is president of the Iowa Board of Regents, which oversees the state's public universities.

In 2011, Rastetter led a group of important Iowans who flew to New Jersey to beg Christie to jump into the race and save the party from Romney.

Now, Rastetter said, he is neutral and plans to stay that way for a while.

"I like and respect a number of the people in the race, and Gov. Christie is one of them," he said. "Iowa needs to meet him and the others, and they'll make the decision."

The situation is different heading into 2016 than it was four years ago, when he and other party leaders feared Romney's ultimate defeat. "I felt we needed Chris Christie to run," Rastetter said. "We have a much broader field than last time around. . . . We need the field to settle itself."