By Roger Harrabin
BBC environment analyst
Senator Obama has supported corn subsidies to farmers
US presidential hopeful Barack Obama is coming under increasing pressure to change his policies on biofuels.
Senator Obama has been a big supporter of corn subsidies for American farmers to produce the plant-fuel ethanol.
But a new report from his own green adviser warns of the many problems associated with the biofuel.
Daniel Kammen's paper says that a car will emit more greenhouse gases driving on corn ethanol processed with coal than it will using normal petrol.
The University of California, Berkeley, professor says the boom in subsidised American corn production is driving up the cost of animal feed - and forcing soy production to Brazil where it creates still more greenhouse gases if it is planted on virgin land.
Professor Kammen says much smarter biofuels policies are needed, but admits that the analysis of this highly complex field will prove very difficult.
"To address this clash of policies, views, economic valuations and environmental goals, a clear set of evaluation methodologies, and high-quality, open access to data will be required," his paper adds.
Senator Obama has previously been a strong supporter of President Bush's corn subsidies which put money into the pockets of his voters in Illinois - the second biggest corn state in America.
He has also said in the past that the subsidies help with energy security and climate change.
Recently, he has been shifting position somewhat but we may not know until after the election how much he has been listening to Professor Kammen.
At the moment, he faces a clear tension between upholding subsidies to the farmers who helped him into the presidential race and upholding his promises on the global stage to tackle climate change and help the poor.
Senator Obama has also backed President Bush's controversial coal-to-liquid fuel programme which benefits coal miners in the south of his home state.
Senator McCain has been consistently sceptical of corn subsidies to farmers.