Last week, Jason Grumet, one of Obama's main energy advisors, caused a small splash in enviro-policy circles by telling Bloomberg News that, if Obama becomes president, his EPA would probably use the authority given to it by the Supreme Court last year and start regulating carbon-dioxide emissions directly. In other words, an Obama administration wouldn't necessarily have to wait for Congress before starting to curtail greenhouse gases.
Now, my sense has is that the Obama camp is simply applying a little pre-emptive pressure to the various interest groups that are going to be involved in climate policy—especially businesses. These folks may not want any sort of carbon cap, but if Obama's EPA is going to start cracking down on greenhouse gases anyway, using the relatively clumsy Clean Air Act, then they'll probably line up to support a cap-and-trade bill. (In theory, a trading regime is more cost-effective than straight-up regulation, since it allows the cheapest and easiest reductions to get done first, though here's some dissent on that point from Gar Lipow.)
But here's a different view, courtesy of Michael Northrup and David Sassoon writing in Environmental Finance, making the case for Clean Air Act regulation on the merits. They don't trust Congress to design a decent cap-and-trade bill, and think that any new trading regime would take too long to get up and running. The catch is that no one agrees on how the EPA would regulate carbon on its own. Would it build off and coordinate existing state-level endeavors like the Western Climate Initiative or RGGI? Would the EPA just set flat CO2 limits on all coal-fired plants, oil refineries, steel and concrete plants, etc? It's not clear, and the EPA is still seeking public comment from experts. But if Obama becomes president and Congress can't pass its own bill on greenhouse gases, this could be the epicenter of climate policy over the next four years, so it's one to watch very carefully.