The legislation would require the country to reduce carbon dioxide and other greenhouse gas emissions that can lead to climate change — by 17 percent by 2020 and about 80 percent by the next century. To do that, electricity producers and industrial plants would have to make a dramatic shift away from the use of fossil fuels through increased efficiency, move toward greater use of renewable energy or pay for ways to capture carbon emissions.

Democrats have sought to limit the economic impact with provisions that would make available pollution allowances to utilities and energy-intensive industries, and protect low-income consumers from higher energy costs by providing them rebates and credits.

But some Democratic lawmakers, from regions where utilities and factories rely heavily on coal, remain worried about higher energy prices and the impact that might have on voters.

Still other Democrats complain the bill has been watered down too much with concessions to garner broader support. Gore was said to be making calls from his home in Tennessee to shore up support among those lawmakers as well as others still on the fence.

Gibbs said Obama was calling lawmakers, too, although he declined to give any names. Carol Browner, the White House coordinator on energy and climate, also was on the phone to Capitol Hill as was chief of staff Rahm Emmanuel.

Lobbying on both sides of the issue intensified this week as Pelosi said she wanted a bill finished before lawmakers left for the July 4 holiday recess.

While most environmental groups as well as a number of business organizations and corporations have endorsed the bill, other industry groups, including the American Petroleum Institute and the U.S. Chamber of Commerce have called for its defeat. Despite the concession to farmers, the American Farm Bureau Federation said Wednesday it remains opposed, calling the bill "seriously flawed."

The bill's sponsors have been horse trading with a succession of Democratic lawmakers.

In a nod to farm groups and the ethanol industry, the bill's sponsors agreed to bar the EPA for at least six years from considering international land use changes when determining whether corn ethanol is a climate-friendly fuel. Environmentalists have argued that corn ethanol emits more greenhouse gases than conventional gasoline if global land use changes as a result of greater corn demand are taken into account.

And farmers were assured more favorable treatment in how pollution "offsets" are managed. These are credits farmers can sell in exchange for planting trees or adopting practices that sequester carbon in the ground.

A provision was included to allow businesses greater access to federal forests by broadening the definition of "biofuels" to include salvage lumber and brush from federal forests.

Rural electric cooperatives, who had argued they were being treated unfairly in the distribution of emission allowances, won an agreement to funnel more allowances their way.

Still the issue of future energy prices and the potential economic — and political — fallout of climate legislation dominated lawmakers' concerns.

"What we see is a job killer. ... There's no question that cap and trade will cost millions of jobs," said House GOP Whip Eric Cantor of Virginia, seeking to rally opposition to the bill. "There's no question the cap-and-trade will cost millions of jobs" and higher energy prices.

A new Washington Post-ABC News poll said three-quarters of Americans think the federal government should regulate the release of greenhouse gases, and 56 percent say they would approve such measures even if it increased their monthly electricity costs by $10.