WASHINGTON, Jan 5 (Reuters Legal) - The Obama administration is facing intense pressure from all sides to delay its efforts to limit greenhouse gases, but don't expect it to call off its chief enforcer on climate -- the EPA -- without a fight.

Congress failed last year to pass a climate change bill and into this vacuum has been thrust the Environmental Protection Agency -- a symbol of government excess for the political right but an institution of last resort for environmentalists.

President Barack Obama has long vowed that the EPA would enforce legal requirements to keep the air clean if Congress was not up to the task. Some lawmakers, mainly Republicans, have vowed to stop the EPA in its tracks.

"Until this issue is settled it will be almost impossible to pass any energy legislation," said Daniel Weiss, of the Center for American Progress Action Fund.

The EPA has already begun regulating emissions with new rules for power plants and refineries that went into effect on Jan. 2. Last month the EPA also released its schedule for more rules over the coming months.

But Obama will have to walk a fine line as the agency set up in 1970 by the Nixon administration faces a rising tide of opposition.

On one hand, the White House must contend with a vocal and increasingly powerful opposition arguing that new regulations will harm a fragile economy. At the same time, it must deal with supporters who say the EPA is legally bound to act and that Obama should stick to his campaign pledges.

"The Obama administration is fighting business and fighting environmentalists," said Kevin Book, an analyst at ClearView Energy Partners, LLC. "They don't have the luxury of being able to neglect their obligations ... to the environmentalists."

CARBON DIOXIDE THREAT

The EPA began moving to regulate emissions causing global warming after the agency designated carbon dioxide a threat to human health and welfare more than a year ago, backed by the Supreme Court's 2007 decision in Massachusetts v. EPA.

Republicans, who now control the House of Representatives in Congress, are moving to block EPA action.

Fred Upton, a Michigan Republican who will head the powerful House Energy and Commerce committee, called the EPA's carbon rules an "unconstitutional power grab" in a recent op-ed for the Wall Street Journal.

Saying that the rules will kill millions of jobs, Upton called on Congress to pass legislation barring the EPA from carrying out carbon regulations until courts have decided on the legality of the rules.

The EPA also faces a slew of lawsuits from states and industries attempting to block the climate regulations, as well as lawsuits from states and organizations pressing the agency to move forward with the rules.

At least 15 states, including Texas, Virginia and Florida, as well as companies and business groups are challenging the EPA over its 2009 "endangerment finding" in which the agency labeled greenhouse gases a threat to human health.

Many of the same groups, including the Coalition for Responsible Regulation, have also sued the EPA over the so-called "tailoring rule," in which the agency applied the Clean Air Act to only the biggest greenhouse gas polluters. The plaintiffs say the EPA does not have the right to apply the law so selectively.

While the Obama administration has said it opposes moves impeding the EPA, the real test will come when such measures are attached to unrelated pieces of "must pass" legislation.

Republicans will likely attempt to tie proposals attacking the climate rules to a major budget bill Congress will need to pass in March to keep the government running.

But that is where the dealmaking could begin. Obama could agree to some delay in the EPA's moves in return for a few concessions.

Similar to the fight over extending the Bush tax cuts, the White House could then say Republicans forced a compromise by holding important legislation "hostage" over the EPA rules.

"If the Republican House essentially starts the process ... the administration is no longer responsible for disappointing the left and no longer forced to essentially impose costly regulations on emitters," Book said.

Pat Parenteau, an environmental law professor at Vermont Law School, said he does not think the White House is going to roll over without winning some climate concessions.

A compromise that postpones some of EPA's climate rules in the short-term, in exchange for a renewable power standard or more energy efficiency measures could produce immediate environmental benefits, he said.

"There could be a deal in there that gets quicker greenhouse gas reductions than you get through the rules," Parenteau said.

An extended fight over climate rules could potentially hamper other administration energy priorities such as promoting natural gas and nuclear power.

The consolidated challenges to the EPA's new rules are Coalition for Responsible Regulation et al v. EPA, U.S. Court of Appeals for the District of Columbia Circuit, Nos. 09-1322, 10-1073, 10-1092, 10-1083, 10-1109, 10-1131, 10-1200. Court filings list Assistant Attorney General Ignacia Moreno and Perry Rosen, Thomas Lorenzen, Jon Lipshultz, Angeline Purdy, Perry Rosen, David Gunter, Eric Hostetler and Kim Smaczniak of the Department of Justice as counsel for the agency alongside EPA lawyers John Hannon, Elliott Zenick, David Orlin, Steven Silverman, Brian Doster, Carol Holmes and Howard Hoffman. Holland & Hart attorneys Patrick Day (Cheyenne, Wyoming); Paul Phillips, Robert Connery, Cori Peterson and Cathy Milkey (Denver); John Bryson and Emily Schilling (Washington, D.C.); and James Holtkamp (Salt Lake City) are listed as counsel for lead petitioner, Coalition for Responsible Regulation, alongside Vinson & Elkins' Eric Groten (Austin, Texas) and John Elwood (Washington, D.C.).

(Reporting by Ayesha Rascoe of Reuters; Additional reporting by Timothy Gardner of Reuters and Terry Baynes of Reuters Legal)