“He doesn’t talk about it very much, and he doesn’t talk about it very convincingly,” she said.

Multiple polls have found climate change has been among the top three issues for Democrats in the 2020 primary, often second only to health care. And many Democratic voters are happy with the front-runner. One Super Tuesday exit poll found that 34 percent of voters who cited climate change as their most important issue went for Mr. Biden, compared to 28 percent who voted for Mr. Sanders.

Mr. Biden said this week he was prepared to act aggressively. After winning three more states that held primaries on Tuesday, he extended an appeal, “especially to the young voters who have been inspired by Senator Sanders: I hear you. I know what is at stake. And I know what we have to do.”

But Mr. Biden has resisted tacking left on climate change these past few weeks in the way he did recently when he endorsed Senator Warren’s bankruptcy plan.

His climate change plan would inject $1.7 trillion into the economy with an aim of achieving zero emissions in the United States by 2050. Mr. Sanders, in comparison, calls for spending $16 trillion and completely eliminating fossil fuels from the American economy by 2050. Asked about it Sunday in a debate, Mr. Biden was unapologetic.

“It is ambitious enough to tackle the crisis,” Mr. Biden said. Noting his home state of Delaware is three feet above sea level and vulnerable to warming, he told Mr. Sanders, “I don’t need a lecture on what’s going to happen about rising seas.”