WASHINGTON - Eight Senate Democrats, including Pennsylvania's Bob Casey, are challenging the Environmental Protection Agency's authority to regulate pollution blamed for global warming.

In a letter written by Sen. Jay Rockefeller of West Virginia, they said the agency lacked the power to restrict greenhouse gases from stationary sources such as power plants, factories, and mines. The senators said Congress, not the EPA, should address an issue with big implications for thousands of U.S. jobs and businesses.

Opposition to EPA regulations by Democrats could pose a serious blow to the Obama administration's effort to restrict heat-trapping greenhouse gases. While the administration is pushing for Congress to pass a comprehensive climate bill this year, officials have not ruled out controlling greenhouse gases through regulation.

Besides Rockefeller and Casey, the letter to EPA Administrator Lisa P. Jackson was signed by Democrats Mark Begich of Alaska, Robert C. Byrd of West Virginia, Sherrod Brown of Ohio, Claire McCaskill of Missouri, Carl Levin of Michigan, and Max Baucus of Montana.

The Democrats said they did not object to EPA regulation of emissions from cars and light trucks but questioned the agency's ability to do anything further under the Clean Air Act. They asked Jackson to clarify the EPA's timetable and suspend any regulations for coal-fired utilities and other industrial facilities until Congress acts on climate and energy legislation.

Jackson said late yesterday that regulation of greenhouse-gas emissions would not begin before January at the earliest. For the first half of 2011, only large polluters that already must apply for Clean Air Act permits will need to address such emissions, she told the senators in a letter.