Donald Trump may have decided to withdraw the US from the 2015 Paris accord—but America’s states are not willing to abandon the landmark climate agreement just yet.

So far, nine states have joined to form the United States Climate Alliance (paywall) to implement the Paris accord’s goal of cutting carbon emissions by as much as 28% at the local level, undermining the US president’s pullout and acting as as a sort of rebellious “shadow government” on the environment. The states—California, New York, Massachusetts, Washington, Connecticut, Rhode Island, Vermont, Oregon, and Hawaii—collectively represent a quarter of the US population and produce 30% of the country’s annual GDP.

They are not alone.

Ten of the remaining 41 states have expressed an interest in joining. The alliance is being helped at the city level by 187 mayors of major cities like Los Angeles, Boston, New York, and Chicago, who call themselves the Climate Mayors.

In addition, the former mayor of New York, Michael Bloomberg, who is currently the UN secretary-general’s special envoy for cities and climate change, said on June 2 that US states, cities and businesses will work without Washington, forming a group called America’s Pledge, which includes mayors, governors, more than 80 university presidents, and more than a hundred businesses.

His charity, Bloomberg Philanthropies, also committed up to $15 million to meet American financial obligations under the agreement. In a statement, the former mayor said, “Americans will honor and fulfill the Paris Agreement by leading from the bottom up–and there isn’t anything Washington can do to stop us.”

There is reason to believe these rogue agents could succeed. About 70-80% of environmental regulation occurs at the state and local level. California led national efforts to reduce vehicle emissions (paywall) when the federal government was unwilling, for example. Bloomberg told the New York Times (paywall) that, with a concerted effort, this fledgling coalition could meet or even exceed former president Barack Obama’s pledge to reduce US greenhouse gas emissions by 26% from 2005 levels by 2025.