Three Democratic senators poised to head committees grappling with global warming pressed President George W. Bush for mandatory U.S. limits on greenhouse gases.

In a letter to Bush on Wednesday, Sens. Barbara Boxer, Jeff Bingaman and Joe Lieberman said voters in the election last week demanded that the government reduce America's heat-trapping greenhouse gases that are contributing to the Earth's warming.

"The recent elections have signaled a need to change direction in many areas, including global warming," the senators wrote.

Boxer, Bingaman and Lieberman will, respectively, head the Senate's environment, energy and homeland security committees when Democrats take control in January.

The White House, however, sent signals that the new Democratic Congress should not expect Bush to budge from his opposition to regulating industrial carbon dioxide. That position he took in March 2001 was a reversal of his campaign stance.

James Connaughton, chairman of the White House Council on Environmental Quality, told reporters Tuesday that the administration has not budged from its belief that regulating carbon emissions would undermine the U.S. economy.

"We still have very strong reservations about an overarching, one-size-fits-all mandate about carbon," he said. Connaughton added that most bills in Congress aimed at cutting emissions of carbon dioxide probably would raise energy prices.

But he said the White House was willing to work with Congress toward raising mileage standards for passenger cars.

The Energy Department reported this week that the United States' greenhouse gases, already a quarter of the world's output, rose again in 2005 by 0.6 percent above 2004 levels. That was a slight improvement when compared with the average annual growth rate of 1 percent since 1990.

Boxer has said her committee's first hearing next year will focus on global warming. One task will be reconciling several competing approaches to global warming.

"We pledge to work to pass an effective system of mandatory limits on greenhouse gases," the three senators wrote Bush. "We urge you to work with us to reach this result and to signal to the world that global warming legislation is on the way."

Kristen Hellmer, a spokeswoman for the White House environmental council, said Wednesday that Congress should approve Bush's plan for more federal research into alternative fuels for motor vehicles, nuclear, solar and wind energy and cleaner coal technologies.

"The president already has in place an aggressive climate change strategy that is realizing results," Hellmer said.

The departing chairman of the Senate environment committee, Republican James Inhofe, promised to lead the opposition to climate bills that pose big economic costs in next year's Senate. Democrats will enjoy a 51-49 majority, but 60 votes are often needed to overcome minority opposition.

"Many of you might be thinking that the Democrats' razor-thin majority means that global warming-inspired carbon cap legislation is somehow now going to sail through the next Congress," said Inhofe, who has called global warming an hysteria-driven hoax. "Well, I can assure you that will not happen."