Maryland Gov. Robert L. Ehrlich Jr. yesterday announced tighter rules aimed at reducing the amount of pollution spewing from the smokestacks of the state's six largest coal-fired power plants.

The requirements, set to take effect next summer, could help Maryland meet most federal air quality standards well ahead of a 2010 deadline. Ehrlich (R) said he believes the rules also will aid in the recovery of the Chesapeake Bay, which he has made a major goal of his administration.

"Today is a historic day in Maryland policy history," Ehrlich told students in the fifth period Advanced Placement Environmental Science class at Walt Whitman High School in Bethesda, where he unveiled his plan.

The governor told them that it was "the most sweeping, most far-reaching" initiative he could offer without crippling two of the state's largest power providers. "Policymakers try to achieve balance," he said. "I believe this is an aggressive but doable plan."

Several environmental leaders and Democratic lawmakers described the governor's initiative as a welcome, if modest, step.

"Look, it's better than not doing anything, and opposing everything. In that regard, I appreciate it and welcome it," said Sen. Paul G. Pinsky (D-Prince George's), whose own fresh-air legislation drew opposition from the governor during the last General Assembly session. "But this is an extremely modest measure. To say this is problem solved, it just isn't."

Pinsky and others said they would renew their push for a bill enforcing more stringent rules, noting that Ehrlich's proposal is a regulation -- more easily changed or relaxed than a state law.

"What's to stop him from reversing himself and getting rid of these rules as soon as people turn their backs?" asked Montgomery County Executive Douglas M. Duncan, a Democrat running for governor who has endorsed Pinsky's approach. "That's why we need a law."

The governor's political supporters said the new clean-air initiative will burnish Ehrlich's environmental credentials in time for his 2006 reelection bid, especially when coupled with the "flush tax" he imposed two years ago to pay for improvements at treatment plants that dump sewage into the Chesapeake Bay.

Under the clean-air plan, three power plants in Montgomery County and Southern Maryland owned by Mirant Corp. and three in Anne Arundel and Baltimore counties owned by Constellation Energy Group will have to reduce emissions of nitrogen oxides, sulfur dioxide and mercury. The companies no longer will be permitted to purchase credits that enable them to bypass those standards.

Ehrlich said the new regulations come "with a serious price tag" for the two companies, estimated in the hundreds of millions of dollars. Constellation released a statement yesterday saying that could mean higher electric bills for Maryland customers. A spokesman for Atlanta-based Mirant said officials there were studying the proposal.

In part, the proposal nudges the utilities in a direction they were being forced to go. During the Clinton administration, coal-burning power plants were classified as a source of toxic mercury and told to reduce mercury emissions by up to 90 percent by 2008. This spring, the Bush administration changed the standards, announcing rules that would have the plants cut mercury emissions by 70 percent by 2018. Ehrlich's proposal is aimed at meeting standards for ozone and fine particle reductions set for 2010, goals established by the Environmental Protection Agency last year for communities that do not meet the nation's smog standards. The District and surrounding counties and cities in Maryland and Virginia were among 474 jurisdictions nationwide that failed to meet those standards.

Fifteen states, and Baltimore, as a separate plaintiff, are suing the U.S. Environmental Protection Agency, pushing for stricter controls on mercury. Virginia is not a party to the lawsuit over mercury rules. Rules on nitrogen oxide, sulfur dioxide and mercury emissions for Virginia "are a work in progress," said Frank O'Donnell, president of Clean Air Watch, a Washington-based watchdog group.

O'Donnell said Ehrlich's proposal "ignores the whole question of carbon dioxide from power plants, which is essentially protecting the power industry from doing its share to deal with global warming, and aligns this plan completely with the Bush Administration on that point."

Maryland Democrats suggested that Ehrlich chose to enact his plan through executive regulations, rather than do battle with the Democrat-controlled legislature over new pollution limits. Such regulations require review by a legislative committee but no vote by the full General Assembly.

Senate President Thomas V. Mike Miller Jr. (D-Calvert) said lawmakers retain the authority to review the governor's plan or even render it irrelevant by passing their own proposal.

"Can we make something good out of this? Absolutely," Miller said. "We can widen it, we can enhance it, we can put it into statute."

Miller said Ehrlich also may have inadvertently breathed new life into legislation that would seek a 90 percent mercury reduction far earlier than 2018, the deadline in Ehrlich's plan, as well as significant carbon dioxide emission restrictions, which go unaddressed in the governor's plan.

But Sen. Sandra B. Schrader (R-Howard) said she does not believe that legislation will be needed.

"By coming out and stepping forward now, he has preempted their argument," she said.

Gov. Robert L. Ehrlich Jr. -- with Walt Whitman student journalists Maddie Koines and Nate Freeman and press secretary Greg Massoni -- described his anti-pollution plan as "aggressive but doable."Gov. Ehrlich, with Secretary of the Environment Kendl P. Philbrick, called his initiative "sweeping" and "far-reaching."