Joshua Orton, a spokesman for Mr. Sanders, said critics were failing to see that momentum is already moving toward dramatic action. The price of renewable energy is falling. Technology is improving. And the public is increasingly demanding action.

“Many critics are working from a set of pessimistic, establishment political assumptions that will, frankly, result in disaster,” he said. “The current model is unsustainable, and it’s wrong to assume that the plan accepts the status quo in terms of the costs and technology of today.”

But to most climate change experts, Mr. Sanders’s promise to reduce American emissions by 71 percent in a decade and to fully convert to renewable energy by 2050 may win high marks for ambition, but major elements of his plan to get there are implausible.

The heart of Mr. Sanders’s plan, a government-run effort to build, manage and distribute renewable energy on a vast scale, would cost more than $2 trillion if it could pass muster in Congress and, several economists said, it might not even curb emissions.

His rejection of nuclear energy and technology to capture carbon emissions would make his carbon-reduction targets much harder. His notion of paying for renewable energy by “making the fossil fuel industry pay for their pollution” is vague. And, economists said, his climate plan fails to consider his larger agenda, such as the new infrastructure projects in his economic plan that would create a burst of new emission s. High-speed rail, wind turbines and mass transit need steel and concrete, the production of which requires energy.

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“He’s trying to set a marker in terms of the pace and scale of spending that he’s proposing,” said Jesse Jenkins, an energy systems engineer and assistant professor at Princeton University. But, he added, “I don’t think that represents a very nuanced understanding of the set of challenges that we face.”