By Valerie Volcovici

WASHINGTON (Reuters) - A Supreme Court decision on Tuesday upholding U.S. rules that curb air pollution that floats across state lines was seen as a boost for the Environmental Protection Agency's upcoming plan to crack down on carbon emissions from power plants.

The top court backed a federal regulation requiring 28 Midwestern and Appalachian states that cause smog and soot-forming emissions to limit pollution from their smoke stacks before it wafts downwind, mostly to eastern states.

The D.C. Circuit court in 2012 had sided with the industry and certain states that said the EPA exceeded its authority by issuing a national plan.

Lawyers said the 6-2 Supreme Court decision to side with the EPA was a timely boost for the agency as it moves to regulate carbon dioxide emissions from the country's power plants using a different section of the Clean Air Act.

David Marshall, senior counsel to the Clean Air Task Force, an environmental group involved in the case, said the court also recognized the flexibility the EPA has when implementing different aspects of the wide-ranging law.

"This could mean that future court decisions will allow EPA some measure of flexibility in implementing sections of the Clean Air Act involving very complex issues, as long as EPA is interpreting the statute in a reasonable and equitable manner," Marshall said.

Howard Learner, executive director of the Environmental Law and Policy Center, added that the high court gave "considerable deference" to the agency’s standard-setting expertise, which will boost the agency as it prepares to issue what might be its most far-reaching regulations to date.

In June, the EPA is expected to unveil a proposal to curb pollution from existing power plants, the centerpiece of President Barack Obama's climate change strategy.

Under the proposal, states would need to submit by 2016 their own implementation plans to meet EPA-set carbon standards or else face a federally-determined plan. A court challenge to that proposal is almost guaranteed.

Learner said that Tuesday's decision, on top of previous high court rulings on greenhouse gas and the recent D.C. Circuit Court of Appeals' decision to uphold EPA rules on mercury and other pollutants set a "strong and consistent precedent for the courts to apply in likely upholding the EPA’s upcoming carbon pollution reduction standards."

While the Cross State Air Pollution rule considered in Tuesday's case and the forthcoming EPA regulations deal with different sections of the Clean Air Act, both call on states to submit tailored plans for meeting a national standard.

"This was a vote for the role of greater control by the EPA, so state autonomy was sort of brushed aside," Jonathan Martel of the law firm Arnold and Porter, who represents industry clients on environmental issues, said of Tuesday's decision.

David Vitter of Louisiana, the top Republican on the Senate Environment and Public Works committee, criticized the ruling.

"This allows (the EPA) to completely ignore the concept of cooperative federalism that requires them to work with states in crafting plans to address air pollution,” Vitter said.

But Vickie Patton, general counsel to the Environmental Defense Fund and an intervener in the case, said the ruling ensured that air quality is protected even if certain states refuse to comply or delay their compliance efforts.

"Today's decision is critically important in ensuring that millions of Americans are protected from smoke stack emissions even when states decline their responsibilities," she said.


(Reporting by Valerie Volcovici, editing by Ros Krasny)