President Barack Obama's push for cleaner-running automobiles is being viewed as the clearest signal yet that he intends to regulate greenhouse gases.

In an ambitious assault on global warming, Obama on Monday directed the Environmental Protection Agency to reconsider previous denials of applications bystates wanting to set their own limits on the amount of greenhouse gases allowed in truck and car exhaust.

For a decade, environmentalists and states have urged the federal government to limit tailpipe emissions — mostly carbon dioxide — that are blamed for global warming.

For almost as long, the Bush administration refused to use existing law to control greenhouse gases, despite increasing scientific evidence that the Earth is warming and court rulings that said the government has the authority to act.

Should the EPA grant California, 16 states and the District of Columbia permission to set a standard for reducing greenhouse gases from automobiles, experts say federal regulations will soon follow, then limits will be ordered for emissions from refineries and industrial plants.

"There is little question that this is heading in the direction of federal regulation of carbon dioxide," said Michael Gerrard, director of the Center for Climate Change Law at Columbia University in New York City.