President Bush says his legislative proposal to cut power plant pollution, submitted five months after he first outlined it, is a market-based system that guarantees cleaner air while keeping electricity affordable.

Democrats in Congress and environmentalists have been portraying the plan, unveiled in February, as a major threat to clean air.

"In the next decade alone, 'Clear Skies' will eliminate 35 million more tons of pollution than the current Clean Air Act, bringing cleaner air to millions of Americans," Bush said in a statement Monday. "And 'Clear Skies' will do this through the use of a market-based system that guarantees results while keeping electricity prices affordable for the American people."

His proposal uses a cap-and-trade system. It would establish a ceiling, or cap, on the amount of emissions from power plants that are major sources of two kinds of dirty air: nitrogen oxide, which causes smog, and sulfur dioxide, which causes acid rain. It also would create the first controls on their releases of mercury.

Utilities that exceeded the limits could purchase credits from other energy producers whose emissions are lower and who choose to sell their ability to pollute -- the unused pollution allowances -- within the cap. The Clean Air Act requires EPA to set national standards and states to implement clean-up plans.

Critics say the plan ignores the ability of some of the dirtiest power plants to avoid emission reductions by buying credits.

"This would be an attempt to undermine enforcement and substitute an industry-friendly emission trading scheme, which we think would actually encourage corporate irresponsibility and be a giant step backward in air pollution control," said Frank O'Donnell, executive director of the Clean Air Trust, an environmental group.

Christie Whitman, administrator of the Environmental Protection Agency, submitted Bush's plan for sponsorship in the Senate by Sen. Bob Smith, R-N.H., and in the House by Reps. Billy Tauzin, R-La., and Joe Barton, R-Texas.

EPA claims the plan can prevent 12,000 premature deaths and tens of thousands of respiratory illnesses a year by 2020, while cutting pollution from power plants by 70 percent.

The Senate Environment and Public Works Committee, on which Smith is the senior Republican member, approved a more expensive rival approach to dealing with air pollution that would regulate heat-trapping carbon dioxide. Smith opposed that.

Sen. James Jeffords, I-Vt., who chairs the committee, said Monday the administration is "ignoring its own warnings on the devastating effects of global warming" by not addressing carbon dioxide in its plan.

Neither approach is likely to win sufficient support to clear Congress this year, however.