The Obama administration is working to forge a sweeping international climate change agreement to compel nations to cut their planet-warming fossil fuel emissions, but without ratification from Congress.

In preparation for this agreement, to be signed at a United Nations summit meeting in 2015 in Paris, the negotiators are meeting with diplomats from other countries to broker a deal to commit some of the world’s largest economies to enact laws to reduce their carbon pollution. But under the Constitution, a president may enter into a legally binding treaty only if it is approved by a two-thirds majority of the Senate.

To sidestep that requirement, President Obama’s climate negotiators are devising what they call a “politically binding” deal that would “name and shame” countries into cutting their emissions. The deal is likely to face strong objections from Republicans on Capitol Hill and from poor countries around the world, but negotiators say it may be the only realistic path.

“If you want a deal that includes all the major emitters, including the U.S., you cannot realistically pursue a legally binding treaty at this time,” said Paul Bledsoe, a top climate change official in the Clinton administration who works closely with the Obama White House on international climate change policy.

Lawmakers in both parties on Capitol Hill say there is no chance that the currently gridlocked Senate will ratify a climate change treaty in the near future, especially in a political environment where many Republican lawmakers remain skeptical of the established science of human-caused global warming.

“There’s a strong understanding of the difficulties of the U.S. situation, and a willingness to work with the U.S. to get out of this impasse,” said Laurence Tubiana, the French ambassador for climate change to the United Nations. “There is an implicit understanding that this not require ratification by the Senate.”

American negotiators are instead homing in on a hybrid agreement — a proposal to blend legally binding conditions from an existing 1992 treaty with new voluntary pledges. The mix would create a deal that would update the treaty, and thus, negotiators say, not require a new vote of ratification.

Countries would be legally required to enact domestic climate change policies — but would voluntarily pledge to specific levels of emissions cuts and to channel money to poor countries to help them adapt to climate change. Countries might then be legally obligated to report their progress toward meeting those pledges at meetings held to identify those nations that did not meet their cuts.

“There’s some legal and political magic to this,” said Jake Schmidt, an expert in global climate negotiations with the Natural Resources Defense Council, an advocacy group. “They’re trying to move this as far as possible without having to reach the 67-vote threshold” in the Senate.

The strategy comes as scientists warn that the earth is already experiencing the first signs of human-caused global warming — more severe drought and stronger wildfires, rising sea levels and more devastating storms — and the United Nations heads toward what many say is the body’s last chance to avert more catastrophic results in the coming century.

At the United Nations General Assembly in New York next month, delegates will gather at a sideline meeting on climate change to try to make progress toward the deal next year in Paris. A December meeting is planned in Lima, Peru, to draft the agreement.

In seeking to go around Congress to push his international climate change agenda, Mr. Obama is echoing his domestic climate strategy. In June, he bypassed Congress and used his executive authority to order a far-reaching regulation forcing American coal-fired power plants to curb their carbon emissions. That regulation, which would not be final until next year, already faces legal challenges, including a lawsuit filed on behalf of a dozen states.

But unilateral action by the world’s largest economy will not be enough to curb the rise of carbon pollution across the globe. That will be possible only if the world’s largest economies, including India and China, agree to enact similar cuts.

The Obama administration’s international climate strategy is likely to infuriate Republican lawmakers who already say the president is abusing his executive authority by pushing through major policies without congressional approval.

“Unfortunately, this would be just another of many examples of the Obama administration’s tendency to abide by laws that it likes and to disregard laws it doesn’t like — and to ignore the elected representatives of the people when they don’t agree,” Senator Mitch McConnell, the Kentucky Republican and minority leader, said in a statement.

A deal that would not need to be ratified by the United States or any other nation is also drawing fire from the world’s poorest countries. In African and low-lying island nations — places that scientists say are the most vulnerable to the impacts of climate change — officials fear that any agreement made outside the structure of a traditional United Nations treaty will not bind rich countries to spend billions of dollars to help developing nations deal with the forces of climate change.

Poor countries look to rich countries to help build dams and levees to guard against coastal flooding from rising seas levels, or to provide food aid during pervasive droughts.

“Without an international agreement that binds us, it’s impossible for us to address the threats of climate change,” said Richard Muyungi, a climate negotiator for Tanzania. “We are not as capable as the U.S. of facing this problem, and historically we don’t have as much responsibility. What we need is just one thing: Let the U.S. ratify the agreement. If they ratify the agreement, it will trigger action across the world.”

Observers of United Nations climate negotiations, which have gone on for more than two decades without achieving a global deal to legally bind the world’s biggest polluters to carbon cuts, say that if written carefully such an agreement could be a creative and pragmatic way to at least level off the world’s rapidly rising levels of greenhouse gas emissions.

About a dozen countries are responsible for nearly 70 percent of the world’s carbon pollution, chiefly from cars and coal-fired power plants.

At a 2009 climate meeting in Copenhagen, world leaders tried but failed to forge a new legally binding treaty to supplant the 1997 Kyoto Protocol. Instead, they agreed only to a series of voluntary pledges to cut carbon emissions through 2020.

The Obama administration’s climate change negotiators are desperate to avoid repeating the failure of Kyoto, the United Nations’ first effort at a legally binding global climate change treaty. Nations around the world signed on to the deal, which would have required the world’s richest economies to cut their carbon emissions, but the Senate refused to ratify the treaty, ensuring that the world’s largest historic carbon polluter was not bound by the agreement.

Seventeen years later, the Senate obstacle remains. Even though Democrats currently control the chamber, the Senate has been unable to reach agreement to ratify relatively noncontroversial United Nations treaties. In 2012, for example, Republican senators blocked ratification of a United Nations treaty on equal rights for the disabled, even though the treaty was modeled after an American law and had been negotiated by a Republican president, George W. Bush.

This fall, Senate Republicans are poised to pick up more seats, and possibly to retake control of the chamber. Mr. McConnell, who has been one of the fiercest opponents of Mr. Obama’s climate change policy, comes from a coal-heavy state that could be an economic loser in any climate-change protocol that targets coal-fired power plants, the world’s largest source of carbon pollution.