WASHINGTON — Hillary Clinton, courting young voters and the broader Democratic base, has promised to one-up President Obama on climate change, vowing to produce a third of the nation’s electricity from renewable sources by 2027, three years faster than Mr. Obama, while spending billions of dollars to transform the energy economy.

A half-billion solar panels will be installed by 2020, she has promised, seven times the number today, and $60 billion will go to states and cities to develop more climate-friendly infrastructure, such as public transportation and energy-efficient buildings. She would put the United States on track to reduce greenhouse gas emissions 80 percent from 2005 levels by 2050. And, she says, she could achieve all that without new legislation from Congress.

But Mrs. Clinton has avoided mention of the one policy that economists widely see as the most effective way to tackle climate change — and one that would need Congress’s assent: putting a price or tax on carbon dioxide emissions.

“It’s possible, theoretically, to do all this without a price on carbon,” said David Victor, the director of the Laboratory on International Law and Regulation at the University of California, San Diego. But, he added, “it’s hard to see how.”