The battle lines on the ethanol in the national gasoline supply debate are getting more and more defined, or at least more legal. The fight between the ethanol industry and Big Oil (represented by the American Petroleum Institute, an association of 500 oil and natural gas companies) has bee winding through the courts, but now API is telling the Supreme Court just how bad E15 is bad for engines.

Here's the recap. The Environmental Protection Agency gave the okay to up the level from E10 (10 percent ethanol mixed with 90 percent gasoline) to E15 for 2007 and newer vehicles back in 2010. Quickly, nine food and farm groups, along with the API, sued the EPA over the decision. The first drops were sold to the public at Midwestern gas stations last summer and, last August, the US Court of Appeals for the District of Columbia decided that the broad coalition could not challenge E15. So, the API appealed the case to the Supreme Court in February.

"E15 could leave millions of consumers with broken-down cars and high repair bills" - American Petroleum Institute.

Here's the latest. This week, the API filed a legal brief filed with the Supreme Court, describing how E15 can damage cars and trucks. Bob Greco, API's director of downstream and industry operations, told the Des Moines Register that, "E15 could leave millions of consumers with broken-down cars and high repair bills. It could also put motorists in harm's way when vehicles break down in the middle of a busy highway." Let's call it corn anxiety.

Ethanol, made most often from corn, can be contentious across the US, with both Maine and Florida taking legislative stands against the biofuel recently. The AFI fight over E15 is the biggest, though, and will likely have wide-ranging consequences for the biofuel's future in the US.