WASHINGTON (Reuters) - A U.S. Senate panel on Wednesday began drawing up a sweeping law that would put mandatory caps on greenhouse gas emissions after a key Republican endorsed the idea.

Sen. Barbara Boxer, California Democrat and chairwoman of the Senate Environment Committee, wants Congress to pass legislation that would for the first time place caps on emissions of heat-trapping gases from power plants, cars and factories.

Those efforts moved forward after Sen. John Warner, a Virginia Republican and one of the most senior committee members, agreed to work with Democrats “to craft a comprehensive climate bill.” The United States is the world’s biggest emitter of greenhouse gases.

Boxer called the accord “a groundbreaking moment,” saying Warner was the first Republican on her committee to back the idea of economy-wide reductions.

“This will be the first time that any committee or subcommittee of Congress has attempted to pass comprehensive climate change legislation,” said Eileen Claussen, president of the Pew Center on Global Climate Change.

A “cap-and-trade” approach -- backed by Warner and Sen. Joe Lieberman, a Connecticut independent, would set industry-specific reduction targets and require those that exceed them to buy permits to pollute.

Coal-burning power plants emit the most U.S. carbon dioxide, about 40 percent of the total, and cars emit about a third of the total.

Boxer said her committee could vote on the plan -- which was still under negotiation -- after Congress returns from its August recess, and the full Senate could vote on it this year.

Boxer’s efforts jibe with those of House Speaker Nancy Pelosi, who wants to halve U.S. emissions by 2050 through a cap-and-trade system.

Rep. John Dingell, chairman of the House Energy and Commerce Committee and a key climate-change negotiator, said on Wednesday the issue would be addressed in “comprehensive climate change legislation” the House will weigh in the fall.

Stabilizing greenhouse gas emissions at a safe level will require a reduction of between 60 percent and 80 percent by 2050, Dingell said.

Dingell’s views are key because he is a long-time ally of Detroit automobile makers, who have resisted attempts in California and other states to regulate vehicle emissions of carbon dioxide.

In a landmark decision, the U.S. Supreme Court ruled this year that the Environmental Protection Administration has the authority under the federal Clean Air Act to regulate greenhouse gases from vehicles.

President George W. Bush opposes mandatory limits on emissions, saying they would harm the economy. Instead, Bush has set a goal of reducing the intensity of emissions -- as measured against economic growth -- by 18 percent by 2012.