The two senators say enforcing a cap would force further research and development of biodiesel and cellulosic ethanol, which don't use food crops. The Corn Ethanol Mandate Elimination Act of 2015 didn't get added to the Keystone XL Pipeline Act recently, so it's now being introduced on its own.
Last month, a group of lawmakers proposed a bill ending the ethanol-blending mandate and proposed to reform the Renewable Fuel Standards (RFS) by putting a cap on ethanol blends at 10 percent. Also last month, Oregon legislators proposed a bill that would eliminate a 10-percent ethanol blend (i.e. E10) throughout the state altogether. Florida was the first state to enact such a mandate by passing an repeal of E10 two years ago.
Ethanol proponents have argued that the biofuel cuts both pollution and US dependence on foreign oil. Opponents say the fuel can drive up food-crop prices and damage some engines, especially small, two-stroke engines.