President Obama on Tuesday called for the first-ever U.S. limits on carbon emissions at existing power plants and vowed to block the Keystone XL pipeline if its construction would exacerbate greenhouse gases, outlining a series of executive actions to combat climate change and igniting a fierce debate over environmental policy in Washington.

“Today, for the sake of our children and the health and safety of all Americans, I’m directing the Environmental Protection Agency to put an end to the limitless dumping of carbon pollution from our power plants and complete new pollution standards for both new and existing power plants,” Obama said in a speech at Georgetown University that will have long-lasting ramifications for his second term.

Obama also surprisingly addressed the construction of the Keystone pipeline, which would carry oil from Canada to Gulf Coast refineries, saying the project’s carbon footprint would dictate his decision. The project is under review by the State Department, and until Tuesday, the president had declined to talk about the major clash inside his administration.

“Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation’s interests,” Obama said. “And our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution.”

With little appetite for climate-change legislation on Capitol Hill, Obama is choosing to press forward with executive actions, mostly coordinated by his EPA, to meet a second-term promise to significantly reduce carbon emissions.

In announcing his plan, which also calls for a doubling of renewable energy production on public lands, Obama took aim at Republicans who say the president’s green agenda will destroy jobs and those who doubt the science behind global warning.

“What you will hear from the special interests and their allies in Congress is that this will kill jobs and crush the economy and basically end American free enterprise as we know it,” Obama said, mocking his detractors.

He later added, “We don’t have time for a meeting of the flat-earth society.”

For their part, Republicans were not moved by the president’s environmental clarion call.

“He may as well call his plan what it is: a plan to ship jobs overseas,” said Senate Minority Leader Mitch McConnell, R-Ky. “Basically, it’s unilateral economic surrender.”

And though environmentalists cheered Obama’s Keystone position, Republicans greeted the president’s declaration as a positive sign for construction of the controversial pipeline.

“The standard the president set today should lead to speedy approval of the Keystone pipeline,” said Brendan Buck, a spokesman for House Speaker John Boehner, R-Ohio.  “Based on the lengthy review by the State Department, construction of the pipeline would not have a significant environmental impact. It’s time to sign off on Keystone and put Americans to work.”

The original State Department analysis said the construction of the pipeline would not significantly increase carbon emissions.

Obama was careful not to outline specific benchmarks for his plan to limit carbon emissions at power plants, wary of likely legal challenges. A senior administration official said the president would call for the EPA to develop emissions standards for power plants by June 2014, with the intent of finalizing the plan in June 2015.

It’s an ambitious benchmark full of uncertainty, as the cap on carbon emissions from existing power plants is expected to trigger a flurry of lawsuits both within the electricity industry and from localities that say the regulations would destroy jobs. There’s no guarantee that such measures would even be implemented by the time Obama leaves office.

Obama has faced mounting pressure from liberals to act on climate change after largely ignoring the issue in his first term and pledging to dramatically reduce U.S. greenhouse gases in his State of the Union and Inaugural addresses.

The president on Tuesday also called for $8 billion in new loan guarantees for green-energy production and enough renewable projects on public lands to power more than six million homes by 2020. And he pressed for stricter efficiency standards for appliances like refrigerators and lamps.

On the international front, Obama vowed to block U.S. funding for new coal power plants overseas unless they used so-called carbon capture technology.