A spokeswoman for Mr. Manchin, Sam Runyon, did not respond to an email seeking comment from him.
However, several congressional Democratic staff members who have been working on the power plant language say they have been in close, frequent contact with Mr. Manchin’s staff as they draft the bill. More than one described the senator’s current stance on the language as “not a no.”
Mr. Biden will need something more definitive than that, however, and soon, if he is to convince other countries that the United States — the world’s largest economy and second-largest greenhouse gas polluter — can change its ways on climate, after four years in which former President Donald J. Trump openly mocked the science of climate change.
In November, Mr. Biden is scheduled to travel to a major United Nations climate change conference in Glasgow, Scotland, at which world leaders are expected to make new pledges committing to stronger reductions of planet-warming emissions — and to demonstrate the domestic policies that will allow them to meet those commitments.
Mr. Biden has already done the former: Earlier this year, he made the bold pledge that the United States would cut its carbon dioxide emissions 50 percent from 2005 levels by 2030. And he has also pledged to eliminate fossil fuel emissions entirely from the power sector by 2035.
It will be extremely difficult, if not impossible, to meet those goals without passage of the Clean Electricity Payment Program, experts say.
“This budget bill needs to pass in order for the United States to have credibility in Glasgow,” said Alice Hill, a senior fellow at the Council on Foreign Relations, who also served as a climate adviser to the National Security Council during the Obama administration.
Speaking specifically of the clean electricity program, she said: “That piece is critical. That is essentially going to be one of the most effective ways to have meaningful reductions immediately.”