President Barack Obama speaks in the East Room of the White House in Washington.

WASHINGTON - President Barack Obama's national plan to combat climate change will include the first-ever regulations to limit carbon dioxide emissions from existing power plants, as well as increased production of renewable energy on public lands and federally assisted housing, environmental groups briefed on the plan said Monday.

In a major speech Tuesday at Georgetown University, Obama will announce that he's directing his administration to allow enough renewables on public lands to power 6 million homes by 2020, effectively doubling the capacity from solar, wind and geothermal projects on federal property. He'll also say the U.S. will significantly expand production of renewable energy on low-income housing sites, according to five individuals briefed on the plan, who were not authorized to discuss it publicly ahead of Obama's announcement and spoke on condition of anonymity.

The far-reaching plan marks Obama's most prominent effort yet to deliver on a major priority he laid out in his first presidential campaign and recommitted to at the start of his second term: to fight climate change in the U.S. and abroad and prepare American communities for its effects. Environmental activists have been irked that Obama's high-minded goals never materialized into a comprehensive plan.

In taking action on his own - none of the steps Obama will announce Tuesday require congressional approval - Obama is also signaling he will no longer wait for lawmakers to act on climate change, and instead will seek ways to work around them.

The lynchpin of Obama's plan, and the step activists say will have the most dramatic impact, involves limits on carbon emissions for new and existing power plants. The Obama administration has already proposed controls on new plants, but those controls have been delayed and not yet finalized. Tuesday's announcement will be the first public confirmation that Obama plans to extend those controls to coal-fired power plants that are currently pumping heat-trapping gases into the atmosphere.

"This is the holy grail," said Melinda Pierce of Sierra Club, an environmental advocacy group. "That is the single biggest step he can take to help tackle carbon pollution."

Forty percent of U.S. carbon dioxide emissions, and one-third of greenhouse gases overall, come from electric power plants, according to the U.S. Energy Information Administration, the Energy Department's statistical agency.