“Together, we are a political and economic force, and we will drive the change that needs to happen nationwide,” Mr. Brown said at a news conference in New York, held as world leaders were gathering for the United Nations General Assembly.

President Barack Obama had pledged that United States greenhouse gas emissions would fall 26 to 28 percent below 2005 levels by 2025. While President Trump has disavowed this goal, the new analysis found that collective emissions in the 14 alliance states are on pace to drop 24 to 29 percent, based on policies already on the books.

The alliance includes California, Colorado, Connecticut, Delaware, Hawaii, Massachusetts, Minnesota, New York, North Carolina, Oregon, Rhode Island, Vermont, Virginia and Washington; plus Puerto Rico. All but two states are led by Democratic governors.

Yet there’s a caveat to this announcement: Because the states in the alliance only represent 36 percent of the nation’s population, the United States as a whole is still expected to fall short of Mr. Obama’s pledge. A previous Rhodium Group analysis estimated that total United States emissions would likely drop just 15 to 19 percent by 2025 as Mr. Trump dismantled federal climate policies.

For the country to meet its commitments under the Paris agreement, further action by states would be needed. The alliance could try to persuade other governors to ratchet up their ambitions, though those prospects are uncertain, since barriers to climate policy in Republican-leaning states are often as much political as technical. Or the alliance states could pursue even deeper cuts themselves. But here, experts say, they may face practical limits on how far they can go to tackle global warming on their own.