A statement he issued seemed designed to send a strong message to his detractors.

“Climate change is real, humans have made a significant impact, and we have the responsibility and capability to address it urgently,” it said. Despite his opposition, Mr. McNamee was confirmed on Thursday, 50 to 49.

Mr. Manchin’s ability to win as a Democrat in a deep-red state has helped hold down the Republican majority in the Senate. But he votes with Mr. Trump 60.8 percent of the time, according to the website FiveThirtyEight. Fewer than half of his votes line up with the positions advanced by the League of Conservation Voters, an environmental advocacy group.

He is also about to lose his two closest ideological compatriots: Senator Joe Donnelly of Indiana and Senator Heidi Heitkamp of North Dakota, Democrats who lost their re-election bids last month. Both of them voted against the confirmation of Justice Brett M. Kavanaugh; Mr. Manchin, bowing to the will of his constituents, was the lone Democrat to vote in favor.

Mr. Manchin’s campaign contributions from coal and energy companies have also drawn the ire of the progressive left. Of the 20 senators who received the most money from coal mining interests in 2018, he ranks 11th and is the sole Democrat, according to the Center for Responsive Politics, which tracks campaign finance.

Alexandria Ocasio-Cortez, the firebrand Democrat and incoming House freshman from New York who has been pushing for a so-called Green New Deal, cited those contributions last week when she expressed opposition to Mr. Manchin.

“I have concerns over the senator’s chairmanship just because I do not believe that we should be financed by the industries that we are supposed to be legislating and regulating and touching with our legislation,” Ms. Ocasio-Cortez told reporters at a news conference with members of the Sunrise Movement, an environmental advocacy group. Movement members also protested outside Mr. Schumer’s New York office on Monday.

Under the seniority system by which Senate Democrats govern themselves, several of Mr. Manchin’s colleagues are ahead of him in the line for the top spot on the energy panel. But for various reasons — including the tradition that a senator serves as the ranking member of only one committee — they seem likely to turn it down. The maneuvering, which will be sorted out before the next Congress begins, is like a game of political musical chairs.