The campaign to curb global warming gained a formidable Senate supporter Wednesday, as Virginia Republican John W. Warner said he will help write legislation to mandate limits on greenhouse gas emissions.

Warner, the ranking Republican on the Senate Environment and Public Works panel with jurisdiction over global warming, said he will join its chairman, Joseph I. Lieberman, I-Conn., in drafting a bill to cap emissions and allow polluters to trade credits.

Warner has long been viewed as a pivotal player in the debate over restricting emissions. He joins John McCain of Arizona and Arlen Specter of Pennsylvania as the third GOP senator to cosponsor carbon-cutting climate-change legislation.

“This is a groundbreaking moment, because Sen. Warner becomes the first Republican on my committee to support an economywide approach to global warming,” said Chairwoman Barbara Boxer, D-Calif.

Lieberman and Warner said they will bring their bill to the panel before the August recess.

“In my 28 years in the Senate, I have focused above all on issues of national security, and I see the problem of climate change as fitting within that focus,” said Warner.

In the House, Energy and Commerce Chairman John D. Dingell, D-Mich., offered new details about a bill he intends to mark up this fall. Dingell said greenhouse gases will have to be cut by 60 percent to 80 percent by mid-century to avert “large-scale” consequences of global warming, a position that lines up with the goals of mainstream environmentalists. Dingell also said it will include a market-based cap-and-trade system.

Jeff Tollefson contributed to this story.