As we've discussed before, the EPA does have the authority to regulate carbon-dioxide under the Clean Air Act. Actually, it's required by law to do so. Details are still being hashed out, but if the Senate fails to pass a climate bill either this year or next, that's a possible Plan B for dealing with U.S. greenhouse gases. But what, exactly, would EPA regulation look like? Dave Roberts has a good, clear piece walking through the specifics. Much recommended.
Quick version: EPA rules might be a workable way to put the kibosh on new coal-fired plants, but it's a convoluted process that could easily get ensnarled by litigation. Plus, the potential for political backlash is high. Oh yeah, and having the EPA tackle carbon (as opposed to Congress) doesn't make for sustainable policy in the long-term—if a Republican enters the White House and decides global warming isn't worth fretting about, it wouldn't be too hard to overturn everything Obama's done. The looming threat of regulations may put pressure on wavering senators to pass their own bill this year, but relying on the EPA to act could be a suboptimal outcome for pretty much everyone.
Update: Hm, interesting. Roberts also interviewed Jason Burnett, who worked on greenhouse-gas rules in the Bush administration. Burnett says it's quite possible for the EPA to set up a carbon cap-and-trade system under the Clean Air Act, without Congress's say-so, by using the legal arguments the Bush EPA made for its industry-friendly trading program for mercury emissions:
It would be, in some ways, a more cumbersome cap-and-trade system than what Congress, at least in theory, could do. [The relevant section of the Clean Air Act, section] 111d is fundamentally a partnership between EPA and the states; EPA can’t set a national program, period, whether it’s cap-and-trade or some other program. Rather, EPA sets out the overall goals and tells the states to figure out how to regulate to meet those goals. The way it would presumably work is, EPA would strongly encourage states to opt in to the national cap-and-trade system—or whatever it develops. But there’s no requirement for states to do that.