WASHINGTON — President Obama, declaring that “Americans across the country are already paying the price of inaction,” announced sweeping measures on Tuesday to reduce greenhouse gas pollution and prepare the nation for a future of rising temperatures.

Embracing wholeheartedly an issue that could define his legacy but is sure to ignite new political battles with Republicans, Mr. Obama said he would use his executive powers to require reductions in the amount of carbon dioxide emitted by the nation’s power plants.

The carbon cuts at power plants are the centerpiece of a three-pronged climate-change plan that will also involve new federal funds to advance renewable energy technology, as well as spending to fortify cities and states against the ravages of storms and droughts aggravated by a changing climate.

Mr. Obama waded more deeply than he has before into the dispute over the proposed Keystone XL pipeline, which would carry heavy crude oil from Alberta to depots and refineries in the Midwest and on the Gulf Coast. He said he would not approve the 1,700-mile pipeline if it was shown that it would “significantly” worsen climate change.