WASHINGTON (Reuters) - Environmentalists hope the push in Congress for climate change legislation is not overwhelmed by the debate dominating Capitol Hill over changing the U.S. healthcare system. But it might be.

Southern Company's Plant Bowen in Cartersville, Georgia is seen in this aerial photograph in Cartersville in this file photo taken September 4, 2007. REUTERS/Chris Baltimore

Already two months behind schedule and unsure whether enough Democrats will play along, Senate leaders still aim to pass a bill by December when a United Nations summit convenes in Copenhagen to set worldwide goals for reducing carbon dioxide and other pollutants.

But as the debate over healthcare legislation rages and with President Barack Obama due to address a joint session of Congress on Wednesday to try to rescue the faltering plan, it was unclear whether rattled lawmakers will have the time -- or the inclination -- to take on climate change.

“It’s not an impossibility, but it’s certainly not a slam-dunk and never has been,” said Frank O’Donnell, president of the activist group Clean Air Watch.

The healthcare debate, O’Donnell added, “has basically sucked all the oxygen out of the room.”

With many moderate Democrats facing a tough vote on healthcare, O’Donnell wondered whether they also would be willing to do so on an environmental bill that could increase consumers’ energy costs. “How many salvos in one year can they take?” O’Donnell asked.

Staffers at the Senate Environment and Public Works Committee have been working behind the scenes on language intended to reel in enough of those moderates.

Senator Barbara Boxer, who heads the committee, has been working off of a bill passed by the House of Representatives intended to cut utility and factory emissions of greenhouse gases by 17 percent by 2020, from 2005 levels. It could be late September or beyond before Boxer is ready to unveil her bill.

Beyond healthcare, the climate bill might have to compete for time with some other major debates, such as new financial industry controls, annual spending bills, U.S. troop levels in Afghanistan and some must-do tax measures.

AVERTING THE “ABYSS”

U.N. Secretary-General Ban Ki-moon on Thursday said the world was “heading toward an abyss” without swift action to reduce carbon emissions. Without it, he told a 155-nation climate conference in Geneva, melting polar ice and rising sea levels will threaten cities from Tokyo to New Orleans.

Environmentalists hope that such high-profile attention will help jolt Congress toward action.

While public support for healthcare legislation has eroded in recent weeks, environmentalists are heartened by polls indicating that voters want Congress to fix global warming by expanding alternative energy sources such as biofuels and solar and wind power.

“Support for energy and climate legislation held firm and ticked up a bit” lately, said Joseph Mendelson, the National Wildlife Federation’s director of global warming policy.

But if the legislation sputters in the Senate, that would not halt Obama’s drive to reduce carbon dioxide pollution.

The U.S. Environmental Protection Agency is considering regulations that would force large polluters -- those spewing at least 25,000 tons annually -- to reduce their emissions.

“EPA can do some important things to start to mitigate greenhouse gas emissions in our country,” EPA Administrator Lisa Jackson said.

Interviewed on Thursday on National Public Radio, Jackson noted that her agency has authority under the existing Clean Air Act, but like Obama, she prefers Congress pass comprehensive legislation.