Ms. Pelosi is likely mindful of her own past mistakes. A decade ago she pushed the last major climate change measure hard, an ambitious bill to cap emissions of climate-warming pollution, then allow industries to trade emissions credits on a pollution credit market. Through force of will, she got the cap-and-trade measure through the House, only to see it die in the Senate without a vote. The next year, Democrats were swept from power, in part because of that vote and the Republicans’ framing of it as a stealth tax hike.

In that Congress, she had a Democratic Senate and a Democratic president, Barack Obama. This time, she has Mr. Trump, a president who calls climate change a hoax, and a Senate in the control of Mitch McConnell, a Republican majority leader from the coal state of Kentucky.

But on Thursday, she was also intent on letting her critics on the left know about her own past efforts, adding that she had made climate change the “flagship issue” of her first speakership.

Proponents of the Green New Deal insisted that Democrats were unified. Both Mr. Markey and Ms. Ocasio-Cortez praised Ms. Pelosi as a “leader” on climate change, and Ms. Ocasio-Cortez said she was not offended by Ms. Pelosi’s description of the Green New Deal in an interview with Politico on Wednesday as the “green dream, or whatever they call it.”

“I think it is a green dream,” Ms. Ocasio-Cortez said. “I will not allow our caucus to be divided by whatever narrative.”

Republicans seized on the proposal with relish, portraying the entire resolution as absurd.

“The socialist Democrats are off to a great start with the roll out of their ridiculous Green New Deal today!” said Bob Salera, a spokesman for the National Republican Congressional Committee, the political arm of House Republicans, who called the idea “zany.”

The Republican National Committee derided it as “a socialist wish list.”

But Democratic candidates for the presidency did not shy away from it once details emerged. Senator Kamala Harris, Democrat of California, quickly sent out a fund-raising appeal, declaring: “For too long, we have been governed by lawmakers who are beholden to Big Oil and Big Coal. They have refused to act on climate change. So it’s on us to speak the truth, rooted in science fact, not science fiction.”