But one of his toughest opponents may end up coming from his own US Midwestern back yard: Agriculture.

Obama on Tuesday forged ahead with landmark moves to remake the US auto industry, forging a coalition of unions and auto executives in an atmosphere of unprecedented financial crisis to retool automakers under a banner of "green cars."

A day before, Congressmen Henry Waxman and Ed Markey introduced a proposed American Clean Energy

and Security Act of 2009, an attempt to reshape energy and environmental policy around a national mission of sustainable long-term prosperity.

But the largest US farm group, the American Farm Bureau Federation (AFBF), immediately panned the 932-page Waxman-Markey bill despite the fact the proposed law "does not include agriculture under the greenhouse gas provisions."

Centerpiece of the proposed law are provisions to put new limits on emissions of carbon dioxide and other greenhouse gas pollution and create a system of tradable "permits" that would reward polluters for changing to green systems.

"The (bill) is laden with so many policy prescriptions that its impact on the US is almost impossible to measure and evaluate," AFBF Bob Stallman said in a statement. "We can be certain of some things, however -- it will increase our operating costs and reduce our competitiveness abroad."

Even as Obama spoke, a new climate study by scientists at the Massachusetts Institute of Technology said global warming's effects this century could be twice as extreme as estimated just six years ago -- which could mean worsening droughts, water shortages, fires and other dire prospects for farmers.

A tortured and uncertain road ahead for US climate change measures, and agriculture's complex role as both a source of greenhouse gasses and potential "sink" for carbon that removes the gasses from the air, got a full airing this week at the World Agricultural Forum industry meeting here.

"We have to have a sense of urgency and have to have serious action on the carbon mitigation challenge," Frank Tugwell, president and chief executive of Winrock International, a consultant on sustainability to rural communities worldwide, told the forum.

"I don't have any confidence that we are going to have legislation that is going to be effective in the United States," Tugwell said at a round-table on climate change.

"The main reason is that the democratic systems are not set up to pay a price in advance for a problem that is this far out. We can't even address Social Security in this country.

"We will not pay a price in advance for something that is hypothetical like this and in the future, even though it is catastrophic for us," Tugwell said.

"We are in a war but we are in a war against an invisible enemy," said Carole Brookins, managing director of Public Capital Partners, a consultant to emerging market governments.

She said voters absorb weather disasters like Hurricane Katrina or droughts without "connecting the dots" on climate.

"People don't want to think about it because ... to think about it is just too many systemic changes," Brookins said.

Mike Walsh, executive vice president of Chicago Climate Exchange, which stands to benefit from a mandatory "cap and trade" law, said putting a price on carbon is a solid first step but agriculture has a long way to go to buy in.

"The question is agriculture and forestry can help solve the problem and get paid for that service. How do we get there? The answer is step by step," Walsh told the forum.

"I'm not as optimistic. You tell me, how do you grow a crop of wheat without using fertilizer? How do you do that without nitrous oxide emissions?" said Allan Buckwell, policy director for Country Land and Business Association in the United Kingdom. "Agriculture is a net emitter."

Tugwell said such objections will have to be overcome. "Agriculture needs to come to the table on climate change, recognizing it is an emitter and focus that it could be part of the solutions," he told Reuters in an interview.