WASHINGTON -- Key Democrats reached a deal Tuesday that its supporters hope will lead to House passage of the biggest environmental bill in decades. The bill would impose nationwide limits on carbon gases emitted from power plants, factories and automobiles.

Farm-state Democrats won concessions that will delay the Environmental Protection Agency from drafting regulations that could hamper the ethanol industry and will hand the Agriculture Department oversight of potentially lucrative projects to reduce greenhouse gases on farms.

The House is expected to take up the legislation on Friday, the first time the chamber will vote on a bill that would impose nationwide limits on the gases blamed for global warming emitted from power plants, factories and automobiles.

The breakthrough came hours after President Barack Obama at a news conference called on the House to pass the legislation, and a new EPA analysis showed that it would raise household energy costs on average only an extra $80 to $111 a year.

"It is legislation that will finally spark a clean energy transformation that will reduce our dependence on foreign oil and confront the carbon pollution that threatens our planet," Obama said. "And that is why I urge members of the House to come together and pass it."

The deal also concludes weeks of closed-door negotiations between the bill's sponsor, Rep. Henry Waxman, D-Calif., and farm-state Democrats, led by Rep. Collin Peterson, D-Minn., who expressed concern in recent weeks that there was not enough in the bill to alleviate the costs for farmers and said they would vote against it.

Peterson said Tuesday the agreement secured his vote. "We have reached an agreement that works for agriculture and contributes to the reduction of greenhouse gas emissions in the United States," he said.

The Obama administration and Congress are under pressure to pass climate and energy legislation prior to an international gathering slated for Copenhagen, Denmark, in December. The U.S. will sit down with other nations to hammer out a new international agreement to curb the emissions linked to global warming.

Peterson and Waxman, chairman of the House Energy and Commerce Committee, announced the agreement late Tuesday. The deal will bar the EPA for five years from including the conversion of forests to crop land when it calculates how ethanol production will contribute to global warming. During that time, the agency will have to conduct a study.

The agreement also includes a promise from Waxman that the Agriculture Department, not EPA, will oversee projects that will reduce greenhouse emissions on farms.

Waxman said after the meeting that he has the votes to pass the bill and the deal with Peterson shows support from three key groups -- environmentalists, farmers and much of the industry involved. Waxman had already struck deals with coal- and oil-state Democrats to ensure their support.

"That's a mighty effective combination," he said.

For weeks much of the behind-the-scenes discussions have focused on how much the bill would cost average consumers, who are likely to face rising prices for electricity and fuels as companies make investments to reduce global warming pollution. Republicans have called the bill a national energy tax that would cost households thousands of dollars a year.

Minority Leader John Boehner, in a memo to House Republicans sent earlier on Tuesday, called the pending floor vote "one of the defining debates of the 2010 cycle" and said that Democrats will vote for it at their political peril.

"Americans know that this bill would have a disastrous impact on our economy and our constituents," Boehner wrote. "The American people will remember this debate and will remember who stands up for them."

But two separate analyses by the nonpartisan Congressional Budget Office offered far lower estimates. The studies said low-income families would receive hundreds of dollars in credits or rebates each year through the sale of permits and the average cost per household would be $175. The EPA analysis released Tuesday said the average household cost would be even lower -- $80 to $111.

The House bill would require factories, refineries and power plants to reduce emissions of carbon dioxide, the leading compound linked to global warming, and six other greenhouse gases by roughly 80 percent by mid-century and hasten the nation's energy shift away from fossil fuels by putting a price on carbon dioxide releases.

The reductions would be made by capping emissions on key polluting sources. The polluters would then be provided emission permits with the cap declining each year. Some 85 percent of the permits would be given away, especially to energy intensive sectors of the economy. But others would be sold by the government, with some of the proceeds used to help people meet higher energy costs.