Lisa Jackson, U.S. president-elect Barack Obama's pick to head the Environmental Protection Agency, pledged Wednesday that decisions at the agency will be based on science and the law and not on politics.

Jackson's opening statement at her Senate confirmation hearing was a veiled criticism of the way the agency was run during President George Bush's tenure. The Bush administration at times ignored the advice of scientific experts on decisions ranging from global warming to air pollution.

"Science must be the backbone of what EPA does," Jackson said. "EPA's addressing of scientific decisions should reflect the expert judgment of the agency's career scientists."

The Senate committee on environment and public works is also considering the nomination of Nancy Sutley, Obama's choice to chair the White House Council on Environmental Quality.

Sutley also vowed to rely on science as she helped to "move the nation to greater reliance on clean energy and increase energy security."

Senators are expected to press both candidates for details on how the incoming administration plans to tackle global warming and water pollution. They also could be asked whether they plan to redo Bush administration rules that Senate Democrats say have weakened environmental protections.

Obama has called for legislation to curb the gas emissions blamed for global warming, but it is unclear whether he will pursue a new law first or use existing statutes to more quickly address the problem. He could immediately grant states like California the right to regulate emissions from automobile tailpipes, or trigger controls under the Clean Air Act.

Democrats will want Jackson, the former head of New Jersey's environmental department, to commit to regulating the disposal of toxic coal ash after two recent spills at power plants in Alabama and Tennessee.

If confirmed, Jackson, 46, would be the first black person to lead the EPA — an agency with 17,000 employees and a $7 billion US budget.

Before running the New Jersey Department of Environmental Protection, Jackson worked at the EPA for 16 years. She served under Carol Browner, former president Bill Clinton's EPA chief and Obama's pick for a new White House position co-ordinating energy and climate policy.

Sutley, 46, is the deputy mayor for energy and environment in Los Angeles. She is the daughter of Argentinean immigrants and is a gay rights activist. She also worked at the EPA during the Clinton administration.

If confirmed, Sutley will co-ordinate energy and environmental policy from the White House.