California and Connecticut Mobilize to Defend U.S. Climate Framework After EPA Repeal

California and Connecticut are preparing a coordinated legal challenge against President Donald Trump’s decision to repeal the federal “endangerment finding,” a landmark determination that greenhouse gas emissions from vehicles pose a threat to public health.

The Environmental Protection Agency last week rescinded the finding, which has long served as the legal foundation for regulating emissions from automakers, power plants, and oil and gas operations. President Trump described the move as the nation’s “biggest deregulatory action.”

Connecticut Attorney General William Tong said the states are developing a multi-state “plan of attack,” carefully assessing legal standing and potential claims before filing suit. California Attorney General Rob Bonta confirmed that discussions are underway and indicated that any legal response would be timely, though precise timelines were not disclosed.

Transportation and power generation together account for roughly half of U.S. greenhouse gas emissions, making the endangerment finding central to federal climate oversight. Legal analysts suggest the repeal could also reopen the door to public nuisance lawsuits, an avenue largely curtailed by a 2011 Supreme Court ruling that assigned primary authority over greenhouse gas regulation to the EPA.

The anticipated lawsuit signals a renewed state-level push to defend climate regulation amid sweeping federal rollbacks.

The California-Connecticut multi-state legal response to the endangerment-finding repeal reflects the well-established pattern of state-level pushback against federal climate rollbacks. The endangerment finding underpins virtually all federal greenhouse-gas regulatory authority; repealing it would reset the legal foundation for climate rules across transport, power and industrial sectors.

Transport and power together representing ~50 percent of US greenhouse-gas emissions plus the possible reopening of public-nuisance litigation pathways captures the substantial regulatory-and-litigation stakes. State attorneys general are experienced climate-litigation participants; their multi-state coordination signals serious pursuit of legal challenges.

For US corporates in transport, power and industrial sectors, financial institutions with US portfolio exposure and their sustainability advisors, Cognitud's ESG strategy and transformation, market intelligence and climate action teams help clients evaluate exposure under evolving federal-versus-state-versus-litigation dynamics, structure decarbonisation strategies robust across regulatory scenarios, and prepare disclosures aligned with SEC, California and ISSB frameworks.