WASHINGTON - The Obama administration is announcing a new pollution rule Monday - and it's expected to be perhaps the most significant US environmental regulation in decades.

But it's not one the White House wanted.

The regulation to limit the pollution blamed for global warming from power plants is a compromise for Obama.

The president again finds himself caught between his aspirations and what is politically and legally possible.

The regulation will provoke a messy and drawn-out fight with states and companies that produce electricity.

Obama is using a 30-year-old law that was not intended to regulate the gases blamed for global warming.

Obama was forced to rely on the Clean Air Act after he tried and failed to get Congress to pass a new law during his first term.