U.S. Senate Majority Leader Mitch McConnell (R-KY) talks to the media after a weekly Senate caucus luncheon on Capitol Hill in Washington March 17, 2015. REUTERS/Yuri Gripas

By Valerie Volcovici

WASHINGTON (Reuters) - The Obama administration's plan for U.N. climate change talks encountered swift opposition after its release Tuesday, with Republican leaders warning other countries to "proceed with caution" in negotiations with Washington because any deal could be later undone.

The White House is seeking to enshrine its pledge in a global climate agreement to be negotiated Nov. 30 to Dec. 11 in Paris. It calls for cutting greenhouse gas emissions by close to 28 percent from 2005 levels within a decade, using a host of existing laws and executive actions targeting power plants, vehicles, oil and gas production and buildings.

But Republican critics say the administration lacks the political and legal backing to commit the United States to an international agreement.

"Considering that two-thirds of the U.S. federal government hasn't even signed off on the Clean Power Plan and 13 states have already pledged to fight it, our international partners should proceed with caution before entering into a binding, unattainable deal,” Senate Majority Leader Mitch McConnell said.

U.S. officials stressed that their Intended Nationally Determined Contribution, U.N. lingo for its official submission, stands on sound legal footing, with the measures drawing authority from legislation such as the Clean Air Act and the Energy Independence and Security Act.

Todd Stern, the lead U.S. climate change negotiator, said he frequently tells foreign counterparts that "undoing the kind of regulation we are putting in place is very tough to do."

But elements of the administration's climate policy already face legal challenges. On April 16, a federal appeals court in Washington, D.C. will hear arguments from 13 states opposed to as-yet-unfinalized regulations from the Environmental Protection Agency (EPA) that target emissions in existing power plants.

And McConnell's warnings echoed the tone of a March 9 "open letter" from 47 Republican senators to Iran, in which they warned a Republican president would not be bound to honor a nuclear agreement struck by Democrat Obama without congressional approval, calling it a "mere executive agreement."

Some observers said that resistance to the administration's climate policies leaves foreign governments questioning whether Obama's commitments can last.

“By strenuously invoking EPA regulations, the Administration is trying to convince skeptical international audiences that the U.S. can actually deliver on its new climate goals, despite Republican resistance,” said Paul Bledsoe, a former Clinton White House official who is now with the German Marshall Fund of the United States.

“But major capitals are likely to remain nervous.”

The administration is clearly sensitive to the threat. Power plants are the biggest domestic source of greenhouse gas emissions, and the EPA is seeking to use its power to slash carbon levels from plants to 30 percent of their 2005 levels by 2020.

On Monday, EPA Administrator Gina McCarthy said the agency had designed power plant rules under the authority of the Clean Air Act - and insisted that they can withstand Supreme Court scrutiny.

"We don’t need a plan B if we are solid on our plan A," she said.

But Jeff Holmstead, a lawyer representing utilities industries for Bracewell & Giuliani and former assistant administrator of the EPA under George W Bush, says even if the courts uphold the EPA proposal on power plants, a future Republican administration can reverse it.

"There are some EPA rules that are very difficult for a new administration to change but this is not one of those rules," Holmstead said. He calculates that at least five high court justices are wary of the EPA's regulatory leeway.

Environmental groups, on the other hand, were more confident that Obama's measures cannot be reversed by the courts or politics.

“The Clean Air Act has proven to be quite durable," said David Waskow, director of international initiatives for the World Resources Institute. "While elements may be slowed or modified by legal challenges, they are rarely overturned.”

(Reporting By Valerie Volcovici; Editing by Bruce Wallace and Grant McCool)