“We were hopeful last year; he was saying all the right things,” Mr. Pica said. “But now he has become a full-blown nuclear power proponent, a startling change over the last few months.”

Image Daniel J. Weiss of the Center for American Progress is disappointed by President Obama's enthusiasm for nuclear power. Credit... Stephen Crowley/The New York Times

Mr. Obama said in his remarks on the nuclear project this week that he knew his policies were alienating some environmentalists.

“Now, there will be those that welcome this announcement, those who think it’s been long overdue,” Mr. Obama said of the new nuclear loan guarantee. “But there are also going to be those who strongly disagree with this announcement. The same has been true in other areas of our energy debate, from offshore drilling to putting a price on carbon pollution. But what I want to emphasize is this: Even when we have differences, we cannot allow those differences to prevent us from making progress.”

Mr. Obama has long supported nuclear power, as a senator and as a candidate for president. Employees of the Exelon Corporation, the Chicago-based utility that is the largest operator of nuclear plants in the United States, have been among Mr. Obama’s biggest campaign donors, giving more than $330,000 over his career, according to the Center for Responsive Politics.

In response to criticism of some of its energy policies, the White House points to its clean energy investments, including $80 billion in stimulus spending on energy-related projects, and its continuing support for comprehensive climate and energy legislation. But critics in the green movement say they wish the president would play a more active role in the climate debate.

“I think we all had higher hopes,” said Bill Snape, senior counsel for the Center for Biological Diversity. “We expected a lot in the first year, and everyone agrees they didn’t quite live up to it. But there is recognition that he and the whole administration will get another stab at it.”