L’AQUILA, Italy  The world’s biggest developing nations, led by China and India, refused Wednesday to commit to specific goals for slashing heat-trapping gases by 2050, undercutting the drive to build a global consensus by the end of this year to reverse the threat of climate change.

As President Obama arrived for three days of talks with other leaders of the Group of 8 nations, negotiators for 17 leading polluters abandoned targets in a draft agreement for the meetings here. But negotiators embraced a goal of preventing temperatures from rising more than 3.6 degrees Fahrenheit, and developing nations agreed to make “meaningful” if unspecified reductions in emissions.

The mixed results underscored the challenges for Mr. Obama as he tries to use his first summit meeting of the Group of 8 powers to force progress toward a climate treaty. With Europe pressing for more aggressive action and Congress favoring a more restrained approach, Mr. Obama finds himself navigating complicated political currents at home and abroad.

If he cannot ultimately bring along developing countries, no climate deal will be effective.

The debate over warming dominated the opening of the summit meeting, but the Group of 8 nations also tackled the global economic recession, Middle East peace, the war in Afghanistan and development in Africa. Mr. Obama invited his colleagues to a nuclear security conference in Washington in March and prepared to announce a $15 billion program to combat world hunger. And in a statement, the leaders said they “deplore postelectoral violence” in Iran, and they pressed Tehran for a diplomatic solution to the standoff on its nuclear program.