AMES, Iowa — Tim Recker has been growing corn in this state his whole life, and using his crops to make ethanol almost as long, at first by the jar for his trucks, now by the barrel for the nation. That is in large part because Congress in 2005 mandated that oil refiners blend ethanol into gasoline.

“When I look out my window and see farms that have built and expanded and improved, it’s because of the ethanol mandate,” Mr. Recker said from his farm in Arlington, Iowa. Mr. Recker, a Republican, said his decision at the presidential caucuses on Monday would be driven by what candidates have said about the 2005 law, which created the Renewable Fuel Standard.

But beyond the borders of a state with outsize importance in the selection of presidents, ethanol may be losing its grip on the body politic. Energy policy experts, advocates in the fight on poverty and even other farmers say a law that has been a boon for Iowa has been a boondoggle to the rest of the country. The ethanol mandate has driven up food costs while failing to deliver its promised environmental benefits. Rising domestic oil production and a global energy glut have all but nullified the pitch that ethanol would help wean the country off foreign oil.

And now a powerful coalition including oil companies, environmentalists, grocery manufacturers, livestock farmers and humanitarian advocates is pushing Congress to weaken or repeal the mandate. As soon as this week, the Senate could vote on a measure to roll back the Renewable Fuel Standard, just days after the Iowa caucuses close and the issue largely goes to rest for another four years.