Senate Majority Leader Mitch McConnell (R-KY), center.
CREDIT: AP Photo / Carolyn Kaster

WASHINGTON — Senator Mitch McConnell of Kentucky has begun an aggressive campaign to block President Obama’s climate change agenda in statehouses and courtrooms across the country, arenas far beyond Mr. McConnell’s official reach and authority.

The campaign of Mr. McConnell, the Senate majority leader, is aimed at stopping a set of Environmental Protection Agency regulations requiring states to reduce carbon pollution from coal-fired power plants, the nation’s largest source of greenhouse gas emissions.

Once enacted, the rules could shutter hundreds of coal-fired plants in what Mr. Obama has promoted as a transformation of the nation’s energy economy away from fossil fuels and toward sources like wind and