WASHINGTON — The Environmental Protection Agency will propose a regulation Monday that will cut carbon-dioxide emissions from existing coal plants by up to 30 percent, compared with 2005 levels, by 2030, according to individuals who have been briefed on the plan.

The draft rule would give states and utilities four options of how to meet the new standard, with different emphases on energy efficiency, shifting from coal to natural gas, investing in renewable energy and discounts to encourage consumers to move to off-peak hours.

The rule represents one of the most significant steps the federal government has ever taken to curb the nation’s greenhouse-gas emissions, which are linked to climate change, and the draft is sure to spark a major political and legal battle.

Conscious of that, President Obama called a group of Senate and House Democrats on Sunday to thank them for their support in advance of the proposed rule, according to a White House official who requested anonymity.

Ever since a climate bill stalled in the Senate four years ago, environmental and public-health activists have been pressing Obama to use his executive authority to impose carbon limits on the power sector, which accounts for 38 percent of the nation’s carbon-dioxide emissions.

Opponents, including coal producers, some utilities and many Republicans, argue that the EPA is using a novel legal approach to demand stringent greenhouse-gas cuts that are not achievable given current technology.

It is unclear whether the proposal ranks as Obama’s most sweeping climate policy, because the plan would cut 500 million metric tons of carbon dioxide by 2030. Previous measures to strengthen fuel-efficiency standards for cars and light trucks will cut 2.9 billion metric tons by 2050.

“This momentous development raises the bar for controlling carbon emissions in the United States,” said Andrew Steer, president of the World Resources Institute.

Steer said that “many states are well-positioned to meet or exceed the proposed carbon reductions through existing infrastructure and policies that are already in place.”

And he said “the proposed standards give states flexibility to implement plans according to their needs.”

The Wall Street Journal first reported details of the rule Sunday afternoon.

Much of the electricity sector’s carbon pollution stems from aging, coal-fired power plants. The average U.S. coal plant is 42 years old, according to the EPA, meaning that most of them aren’t nearly as efficient as new ones, though many have been updated. Some were built when Dwight Eisenhower was president, according to Exelon chief executive Christopher Crane.

The regulations could also affect natural-gas-fired power plants, which emit about half as many greenhouse gases as coal plants. The EPA said that natural-gas-fired combined cycle plants in the United States are 14 years old on average.

The EPA plan resembles proposals made by the Natural Resources Defense Council, which would allow states and companies to employ a variety of measures – including new renewable-energy and energy-efficiency projects “outside the fence,” or away from the power-plant site – to meet their carbon-reduction target. This approach aims to keep consumer electricity prices from rising too sharply as a result.

Usually when the EPA regulates pollutants under the Clean Air Act, the agency sets an emissions limit for each facility. By contrast, under a “mass-based system,” states would have to meet an overall target for greenhouse-gas emissions and ensure that power plants either make those reductions at their facilities or finance efforts to achieve them in other ways, such as by reducing consumer energy demand or investing in carbon-free electricity generation.