Across the country, the burden will be unequally spread, with states that depend most heavily on coal-fired power facing the biggest lift. Today, coal-fired power accounts for about 40 percent of the nation’s electricity over all, but for some states, coal supplies much of the power, while others use very little. In Kentucky, for example, 92 percent of electricity comes from coal. Coal powers 83 percent of Missouri’s needs and 67 percent of Ohio’s. But the West Coast states, which rely heavily on hydroelectric power and other low-carbon sources, get less than 10 percent of their power from coal.

In California, which has already enacted an ambitious state-level cap-and-trade law to reduce carbon pollution, on top of another state law requiring generation of renewable electricity, utilities anticipate that meeting the federal regulation will simply be a continuation of business as usual.

“California is already on track to achieve the reductions in the rule,” said Melissa Lavinson, vice president for federal affairs at Pacific Gas and Electric. “The way we’re moving forward under the California law, we’ve reduced emissions, increased renewables, and we haven’t had a problem with reliability.”

E.P.A. officials say they are aware of the concerns about reliability, particularly in the coal-dependent Midwest.

“There is no way that E.P.A. is going to finalize this rule without being assured that the system will be reliable and cost-effective,” said Gina McCarthy, the E.P.A. administrator. “We are working with utilities on what needs to be tweaked.”

A report issued in February by the Analysis Group, a consulting firm based in Boston, concluded that there were ways for states to avoid blackouts and brownouts during the power transition. The report found, if states were to adopt interstate cap-and-trade plans, along the lines of the program in place in California, they could cut pollution while keeping the lights on.

Under such a program, a cap would be set on total pollution over a region of states, and owners of polluting power plants would pay for permits to pollute. If such programs were spread across some states with many polluting coal plants, and some states with none, it could ease the burden on heavily polluting states. Such an interstate program is already in place in the Northeast.