The 5-4 decision, a victory for environmentalists, found the EPA did not go too far when it overruled a decision by Alaska regulators, who wanted to let the operators of a zinc and lead mine use cheaper anti-pollution technology for power generation.
The four justices who dissented said the ruling undercut the states' power to control their environmental policies.
"This ruling is a victory for environmentalists but they shouldn't count their chips just yet," says CBS News Legal Analyst Andrew Cohen. "There are plenty of other environmental cases to be decided this term, and this ruling doesn't suggest that the others will come out the same way."
The Alaska case was the first of eight environmental cases on the court's docket this term, an unusually high number. The fight was over whether the Red Dog Mine must use equipment that would reduce pollution from a new generator by 90 percent. The state wanted to allow the mine operator, a major employer in a particularly rural area of Alaska, to use equipment that would only reduce pollution by 30 percent.
The Clean Air Act allows state officials to make some decisions involving facilities within their borders, but still gives the EPA wide authority to enforce the anti-pollution law passed by Congress in 1970.
"We fail to see why Congress, having expressly endorsed an expansive surveillance role for EPA," elsewhere in the law, "would then implicitly preclude the agency from verifying substantive compliance," with the portion of the law at issue in this case, Justice Ruth Bader Ginsburg wrote for the majority.
Ginsburg's usual allies on the court's ideological left joined her in the ruling: Justices John Paul Stevens, David H. Souter and Stephen Breyer. The crucial fifth vote came from Justice Sandra Day O'Connor, who usually votes with the court conservatives in states' rights cases.
"You can just tell from the voting and from the language of the ruling that the Court is as divided as ever on the issue," says Cohen. "You also can tell that once again, Justice Sandra Day O'Connor is the swing vote, and that is a fact that is going to dictate how lawyers in the other environmental cases prepare for and execute their oral arguments."
The four dissenters argued that the decision undercuts states' power to control their environmental policies.
"This is a great step backward in Congress' design to grant states a significant stake in developing and enforcing national environmental objectives," wrote Justice Anthony M. Kennedy, joined by Chief Justice William H. Rehnquist, and Justices Antonin Scalia and Clarence Thomas.
"After today's decision, however, a state agency can no longer represent itself as the real governing body. No matter how much time was spent in consultation and negotiations, a single federal administrator can in the end set all aside by a unilateral order," Kennedy wrote.
The case is State of Alaska v. U.S. Environmental Protection Agency, 02-658.