Writing for four justices, Justice O'Connor reasoned that the mandate worked an uncompensated takings by imposing a severe, retroactive, and unexpected financial liability on mining companies.

The question in Eastern Enterprises v. Apfel, No. 97-42, is whether the Government can constitutionally impose retroactive liability on the coal industry for health benefits for retired miners.

So the only question, had the Supreme Court ruled the other way, was one of retroactive liability.

Republicans in both the House and Senate are proposing changes that would repeal the system of retroactive liability that makes polluters help pay to clean up the areas they polluted decades ago.

The Court today overturned a ruling by the Supreme Court of Virginia, which had found that the state did not have any retroactive liability.

Early proposals are already being floated to redefine what constitutes a wetland worth protecting, and to repeal the retroactive liability forcing polluters to clean old toxic waste sites.

He favors ending retroactive liability and paying for Superfund cleanups with a broad-based tax that recognizes the polluted sites as a problem for society, not just insurers.

This would replace Superfund's retroactive liability with a broad-based fund, paid into by businesses across all economic sectors.

It was not clear how broadly the decision would expose other states to retroactive liability for invalidated tax laws.

The Court struck down a 1992 law that required companies previously in the coal mining business to assume retroactive liability for health benefits for miners and their families.