Ezra Klein makes the interesting point that much of the biggest impact of an Obama reelection would not be new policy initiatives, but implementing policy that is already in the pipeline. The biggest example is health care - reelect the President, and it gets implemented. Elect Romney, and repeal becomes much more likely. Klein also cites changes to the tax code.
I'd argue much the same is true with carbon policy. Whatever party controls the Senate, Congress is highly unlikely to pass new climate legislation in its next term. That leaves the Clean Air Act as the only realistic path for climate policy. If Obama is reelected, fuel economy standards for new vehicles out to 2025 will stay in place, performance standards that ban new coal power without carbon capture-and-storage will be finalized (maybe before the election), and standards requiring emissions cuts from existing power plants will probably be put in place as well. Only the last of these is a new policy, though, and it's legally required once the new-source standards are issued.
If Romney is elected, the vehicle standards may stay in place, since the auto companies and Congress broadly support them. But all of the other pieces are likely to be slowed and possibly tossed entirely. Environmental groups (and maybe states) will sue - or, more accurately, resurrect previous lawsuits to force this action that EPA has settled. But the administration will be able to delay action for a long time, and even if it is forced to act, may do so far less stringently.
More broadly, a President Romney might sign legislation overturning Massachusetts v. EPA by stripping carbon out of the Clean Air Act entirely. Even without Republican control of the Senate, support from coal-state Democrats could mean a presidential veto all that stands in the way of such legislation.
None of this would necessarily be bad if a Romney administration put into place some alternative carbon policy. Given his past record on the issue and his relatively moderate statements (by Republican standards) on climate change this week, I suspect that he might sign such a bill, though he wouldn't spend political capital pushing for it. But, as noted, Congress will consider no such action in the next term.
There's therefore a big difference in the climate policy you could expect to see under the different candidates even if you assume Obama could get no new policy past Congress in his second term. Under a President Romney, emissions might go down, but it would almost certainly be due entirely to what my RFF colleagues call "secular trends," not policy.
