On Tuesday, however, 28 states, scores of power companies and industry groups were before the Court of Appeals for the District of Columbia to challenge the E.P.A.’s authority to use this approach under the Clean Air Act, on which the rule is based. Opposing them were 18 states that support the rule and dozens of environmental and public interest groups, as well as the Obama administration.

There is no question that the Clean Air Act allows the federal government to regulate carbon dioxide emissions; the Supreme Court has said it does. And there’s no question that the government can regulate these emissions from power plants; the Supreme Court has said so.

The issue is whether the E.P.A. can use an approach to regulating greenhouse gases, different from the way it has regulated other types of pollutants, like mercury emissions. Traditionally, in reducing those pollutants, the agency has required power plants to make technological improvements. But no economically feasible technology is yet available to control carbon dioxide emissions at the plant level.

The agency has therefore told the states that to meet their targets they must move “outside the fenceline” of existing power plants by, say, switching to cleaner fuels, investing in alternative forms of energy like wind and solar power and setting up cap-and-trade systems with other states. Opponents say this is a prime example of “executive overreach.” But this pragmatic strategy is entirely consistent with the Clean Air Act’s fundamental mandate to seek “the best system of emission reduction.”

The opponents also offer other objections, including a long-forgotten legislative glitch that occurred when Congress amended the Clean Air Act in 1990. The House version of the legislation said the agency could not regulate a new pollutant under one section of the law (the one it is relying on now) if it were already regulating other pollutants under another section. The Senate, in effect, said that such overlapping regulation was fine. The argument that this clerical discrepancy should keep the agency from regulating carbon dioxide is preposterous.