Democratic presidents have tried but failed to enact climate change legislation since the Clinton administration. During a year of record and deadly droughts, wildfires, storms and floods that scientists say are worsened by climate change, Democrats had hoped to finally garner enough political support to enact a strong climate law, even as scientific reports say that the window is rapidly closing to avoid the most devastating impacts of a warming planet.

A major scientific report released in August concluded that countries must immediately shift away from burning fossil fuels in order to avoid a future of severe drought, intense heat waves, water shortages, devastating storms, rising seas and ecosystem collapse. To avert catastrophe, scientists say nations must keep the average global temperature from increasing 1.5 degrees Celsius above preindustrial levels. But as countries continue to pump carbon dioxide into the atmosphere, the average global temperature has already risen by about 1.1 degrees Celsius.

Even as Ms. Pelosi vowed in San Francisco to protect those climate provisions, at least four people in Washington close to the negotiations called the clean electricity program “dead.”

Senator Tina Smith, Democrat of Minnesota and the chief author of the program, said that while dropping it might win Mr. Manchin’s vote on the budget bill, it could cost hers — and those of other Democrats focused on the environment.

“We must have strong climate action in the Build Back Better budget,” she said. “I’m open to all approaches, but as I’ve said, I will not support a budget deal that does not get us where we need to go on climate action. There are 50 Democratic senators and it’s going to take every one of our votes to get this budget passed.”

Mr. Manchin, who has personal financial ties to the coal industry, had initially intended to write the details of the program as the chairman of the Senate Committee on Energy and Natural Resources. Mr. Manchin was considering a clean electricity program that would reward utilities for switching from coal to natural gas, which is less polluting but still emits carbon dioxide and can leak methane, another greenhouse gas. Mr. Manchin’s home state, West Virginia, is one of the nation’s top producers of coal and gas.

But in recent days Mr. Manchin indicated to the administration that he was now completely opposed to a clean energy program, people familiar with the discussions said.