A letter sent by lawmakers to Environmental Protection Agency Administrator Christine Todd Whitman called the new rules "extremely troubling."

"The administration is trying to gut a critical section of the Clean Air Act," Lieberman told reporters.

Edwards threatened to tack a rider onto EPA's active 2003 appropriations bill if the agency presses ahead with the rules before studying public health impacts.

"We are going to consider whatever vehicles we can," Edwards said.

Sen. Kit Bond of Missouri, the top Republican on the Senate Appropriations subcommittee with EPA jurisdiction, decried the possible rider.

"There's absolutely no need to put a purely partisan and politically motivated rider on the ... bill," Bond told reporters.

The letter criticized EPA for pressing ahead on the new rules without a human health study, citing testimony by EPA Assistant Administrator Jeffrey Holmstead at a July 16 Senate hearing. At the hearing, Holmstead "admitted" that EPA will proceed "without having conducted a full analysis of their impact," the letter said.

EPA spokesman Joe Martyak refuted that claim, saying Holmstead indicated that the "EPA has developed an extensive analysis supporting these rules," which will all be made public before the rule is finalized.

A utility lobbying group that supports changing the utility rules dismissed the letter as a political ploy.

"The real force at play here are politics, politics, politics," said Frank Maisano, a spokesman for the Electric Reliability Coordinating Council.

HONING CAMPAIGN ISSUES

Regardless of who emerges as the Democratic presidential front-runner, environmental issues should play prominently in the campaign, Lieberman said.

Green issues are "fair and they should be a part of this year's campaign," Lieberman said, citing what he called the administration's "abysmal" environmental record.

"The regulators have sided with those who they are supposed to regulate," he said.

"Senate Democrats - and the presidential candidates among them - clearly see an opportunity," said Phil Clapp, president of the National Environmental Trust.