WASHINGTON -- The cornerstone of President Barack Obama's plan to address climate change is still months away from being finished, but it's already facing a high-stakes legal challenge from critics who want to halt the process in its tracks.

A federal appeals court hears arguments Thursday in two cases challenging the Environmental Protection Agency's far-reaching proposal to cut Earth-warming pollution from the nation's coal-fired power plants.

The lawsuits -- one from a coalition of 15 states and another brought by Ohio-based Murray Energy Corp., the nation's largest privately held coal mining company -- are part of a growing political attack from opponents who say the move is illegal and will kill jobs, cripple demand for coal and drive up electricity prices.

At issue is whether the EPA has legal authority for its plan under the Clean Air Act. But the agency and environmental advocacy groups have urged the court to throw the cases out as premature, saying legal challenges must wait until the EPA issues a final rule until this summer.

Opponents concede it's not typical for a court to provide relief before a rule is final, but argue that states and the coal industry already face the prospect of shutting down coal plants and spending other resources in anticipation of the rule.

All three judges on the panel hearing the case were appointed by Republican presidents.

The EPA rule proposed last year requires states to cut carbon emissions by 30 percent by 2030, giving customized targets to each state and leaving it up to them to draw up plans to meet the targets.

But a backlash has been building. Last month, Senate Majority Leader Mitch McConnell, a Kentucky Republican, sent a letter urging the governors of all 50 states to defy the EPA by refusing to submit the compliance plans.

West Virginia and other states argue that the plan is illegal because coal-fired power plants already are regulated under a separate section of the Clean Air Act. They say the law prohibits "double regulation."

The legal debate focuses on dueling provisions added by the House and Senate to the Clean Air Act in 1990. The EPA says it wins under the Senate language, but opponents argue the House version should prevail.