Little over a year ago, ethanol was winning the hearts and wallets of both Main Street and Wall Street, with promises of greater U.S. energy independence, fewer greenhouse gases and help for the farm economy. Today, the corn-based biofuel is under siege.

In the span of one growing season, ethanol has gone from panacea to pariah in the eyes of some. The critics, which include industries hurt when the price of corn rises, blame ethanol for pushing up food prices, question its environmental bona fides and dispute how much it...