Senator John Kerry, Democrat of Massachusetts, co-sponsor of the Senate legislation, said he believed that the president’s actions would give a boost to the Copenhagen talks and help move the Senate bill. He called the decision to declare an American target a “game changer,” domestically and internationally.

“By announcing a provisional target, contingent on the support of Congress, the president has defined a path to an international agreement that challenges the developed and developing nations to fulfill their obligations,” he said. “It lays the groundwork for a broad political consensus at Copenhagen that will strip climate obstructionists here at home of their most persistent charge, that the United States shouldn’t act if other countries won’t join with us.”

But Senator James M. Inhofe, the Senate’s most outspoken skeptic on climate change, said that Mr. Obama’s public pledge would do little to speed an international agreement and foolishly prejudged the outcome of a Senate debate that had barely started. Mr. Inhofe, Republican of Oklahoma, said that Senate climate legislation was “dying on the vine” and that the Senate would never ratify a treaty that did not require strong emissions reductions from major developing countries.

“The U.S. Senate has made clear on numerous occasions that unilateral action by the United States is unacceptable, because it will harm our economy and have virtually no effect on climate change,” Mr. Inhofe said.

Mr. Obama takes little risk in appearing briefly at the Copenhagen conference because he and other world leaders punctured expectations for the session 10 days ago in a side meeting of leaders of Pacific nations. The leaders agreed that they would work at Copenhagen toward an interim political declaration on climate change that stopped short of a binding international treaty. Delegates are expected to pledge to complete work on a treaty next year.

Mr. Obama came to office promising to end eight years of relative inaction on climate change under the Bush administration, but the inaction of Congress has limited the administration’s ability to negotiate with other nations. At the Kyoto climate conference in 1997, the Clinton administration joined other industrialized nations in pledging to reduce greenhouse gas emissions by 5.2 percent by 2012, but Congress refused to ratify the agreement because it made no demands of developing nations.

Many foreign leaders, particularly those in European nations that have been more aggressive in dealing with climate change, have become critical of Mr. Obama’s seeming passivity on the issue. The White House appears to hope that the announcement of the targets and the trip to Copenhagen will quiet some of the dissension and help Mr. Obama re-establish American leadership on what he calls one of the signature issues of the time.