On Monday, President Barack Obama directed the Environmental Protection Agency to reconsider a waiver request by California and 13 other states to set standards on auto emissions – including those of greenhouse gases – that are tougher than federal standards.

The directive represents a sharp break from the climate policies of Mr. Obama's predecessor, as well as an embrace of states' rights, a concept not strongly associated with the Democratic party for almost 50 years.

California spent the better part of the Bush presidency trying to set limits on tailpipe emissions of carbon dioxide and other heat-trapping gases. In 2002, it drafted a law that would cut greenhouse gas emissions from new cars sold in the state by 22 percent by 2012 and 30 percent by 2016. Since then, it has been battling the federal government to allow it to adopt this law.

Historically, California has been allowed to adopt emissions standards that are stricter than those of the rest of the country. The state was exempt from the Clean Air Act of 1970 because it already had its own emissions laws in place. Today, US states can choose to follow either federal or California emissions standards, but they are not allowed to create their own standards. Each time California wants to adopt a new clean-air law, it must get a waiver from the EPA, which has never been denied.

Until this one. In December 2005, after finalizing the details of its emissions rules, California submitted its waiver request to the EPA. It didn't hear back from the agency. After two years of delay, in November 2007 California sued the agency, and their suit was joined by 13 other states. The following month the EPA officially denied California's application, with the agency's administrator, Stephen Johnson, saying that he wanted to avoid a "confusing patchwork of state rules."

In May 2008, following a Congressional hearing on political interference in the EPA, Congressional Democrats released a report [PDF] saying that the waiver had unanimous support among EPA career staff and that Mr. Johnson himself seemed to support the idea, but that he changed his position after hearing from the White House. Johnson maintains that the decision was his alone.

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