“A pound of carbon reduced anywhere ought to be treated the same,” he said.

Still, he said, in the draft rule, the nuclear industry fared better than hydroelectricity, the second-biggest source of carbon-free electricity, which got zero credit.

Mr. Mariotte of the Nuclear Information and Resource Service and other nuclear opponents appear to be mostly concerned that the reactors, which they regard as unsafe, could get a new lease on life. His group and others sent a letter to the E.P.A. complaining about a related issue: that the draft carbon rule requires that states with reactors under construction continue building them, or face even harder-to-meet carbon targets.

In fact, the question of where to draw the baseline — that is, the date from which the calculations start — is common to many aspects of the carbon rule.

Senator Bob Casey Jr., a Pennsylvania Democrat, made a similar point in a letter to the E.P.A., saying that the rule should take into account previous work to promote clean energy sources, and to maintain existing nuclear plants.

And at Exelon, a Chicago-based company that is considering whether to retire several reactors in the Midwest, Kathleen L. Barron, senior vice president for regulatory affairs, complained at a Washington conference on electricity and carbon, “We’re essentially taken for granted.”

Assumed in the E.P.A.’s planning is that the five reactors currently under construction in the United States will go online. A twin-reactor Vogtle plant is planned near Augusta, Ga., and another twin-reactor plant is in development in South Carolina. In Tennessee, a reactor that the Tennessee Valley Authority started 40 years ago is almost complete.

They are each multibillion-dollar projects, and certain to raise electricity prices, but because work began before the rule was proposed, the carbon-cutting goals set by the E.P.A. for those states do not include the reactors.