Ambinder explains some political maneuvering:
[A]s the Copenhagen climate talks begin, the Environmental Protection Agency plans to issue a formal "endangerment" finding for carbon dioxide and other greenhouse gases. In doing so, the agency is giving the administration what amounts to a cattle prod. Having "found" that CO2 is a "public danger," and having taken the requisite administrative steps, the executive branch now believes it has the power to unilaterally impose carbon and greenhouse gas emissions caps on industry in the United States. This overhanging boot will threaten to drop until and unless Congress acts. It's a neat executive weapon to have -- one that, incidentally, the Bush administration chose not to take out of the locker, and one that the Obama administration decided to unsheathe as the President prepares to travel to Copenhagen.
Bradford Plumer speculates:
As The Wall Street Journal reports, many large businesses, particularly power companies, hate the idea of the EPA issuing its own carbon rules. They'd rather have Congress set up a more flexible cap-and-trade system. (Electric utilities, in particular, were able to shape the House bill more to their liking, and they'd have less luck with EPA regs.) So will those companies start ratcheting up the pressure on lawmakers to pass a climate bill?