The rule, which is expected to be final next year, will set the first-ever national limits on carbon dioxide, the chief gas linked to global warming from U.S. power plants. They are the largest source of greenhouse gases in the U.S., accounting for about a third of the annual emissions, and make the U.S. the second largest contributor to global warming on the planet.

The regulation is a centerpiece of President Barack Obama’s plans to reduce the pollution linked to global warming, a step that the administration hopes will get other countries to act when negotiations on a new international treaty resume next year.

Environmental Protection Agency data shows that U.S. power plants have reduced carbon dioxide emissions by nearly 13 percent since 2005, or about halfway to their goal. But with coal-fired power plants already beleaguered by booming natural gas supplies and other environmental regulations, experts on Sunday said getting there won’t be easy. The EPA is expected to offer a range of options to states based on where they get their electricity from and how much carbon dioxide they emit in the process.

Obama has already tackled the emissions from the nation’s cars and trucks, announcing rules to reduce carbon dioxide emissions by doubling fuel economy. That standard will reduce carbon dioxide by more than 2 billion tons. The power plant proposal will prevent about 650 million tons of carbon dioxide from reaching the atmosphere, based on the 30 percent figure.

The EPA refused to confirm the details of the proposal Sunday. People familiar with the proposal shared the details on condition of anonymity, since they have not been officially released.

The details were first reported Sunday by The Wall Street Journal.

As Obama prepares to announce the tougher new air quality standards, lawmakers in several states already are trying to blunt the impact on aging coal-fired power plants that feed electricity to millions of consumers.

The push against Obama’s new carbon emission standards has been strongest in some states that have large coal-mining industries or rely heavily on coal to fuel their electricity. State officials say the new federal regulations could jeopardize the jobs of thousands of workers and drive up the monthly electric bills of residents and businesses.

It remains to be seen whether new measures passed by the states will amount to mere political symbolism or actually temper what’s expected to be an aggressive federal effort to reduce the country’s reliance on coal. But either way, states likely will play a pivotal role, because federal clean air laws leave it up to each state to come up with its own plan for complying with the emission guidelines.

The proposed EPA rules to be announced Monday could be the first to apply to carbon dioxide emissions at existing power plants. Coal is the most common fuel source for the nation’s electricity and, when it’s burned, is a leading source of the greenhouse gasses that trap heat in the atmosphere and contribute to climate change.

Without waiting to see what Obama proposes, governors in Kansas, Kentucky, Virginia and West Virginia have signed laws directing their environmental agencies to develop their own carbon emission plans that consider the costs of compliance at individual power plants. Similar measures recently passed in Missouri and are pending in the Louisiana and Ohio legislatures.

Missouri lawmakers went even further in their defense of the coal industry. When activists proposed a ballot initiative barring local tax breaks for St. Louis-based Peabody Energy, state lawmakers quickly passed a measure banning such moves.

Some states have specifically empowered local regulators to develop emission plans that are less stringent than federal guidelines. According to measures passed recently, the state policies are to take into account the “unreasonable cost” of reducing emissions based on a plant’s age and design and the “economic impacts” of shutting down particular power plants.

“The concern is that the federal standards _ if they come out the way that most people expect them to _ are going to drive the cost of electricity up for every single consumer in the state,” said Missouri state Rep. Todd Richardson, a Republican.

Federal emission regulations already allow flexibility for states if they can demonstrate costs would be unreasonable for particular facilities. But a spokesman for the EPA’s Midwestern region, which oversees several states that rely predominantly on coal for their electricity, said he’s unaware of that provision ever being used.

It’s unlikely that the Obama administration would essentially undercut its new carbon emission standards by granting widespread exceptions, said Bill Becker, executive director of the National Association of Clean Air Agencies, which represents air pollution control agencies in 42 states and 116 metropolitan areas.

If a state doesn’t comply with EPA guidelines, the federal agency can create its own plan for the state.

This is not a standard that a state then can willy-nilly ignore,” Becker said. “It’s going to have to achieve at least that standard or more. Period.”