The Washington Post (here) asked its expert panel the following question: "Given gridlock in Congress over the climate bill, is the Obama Administration's fallback strategy to let the EPA regulated greenhouse gas emissions a good idea?"
Only one of the responding experts gave the correct answer to that question, which is that it's the wrong question! Having made an "endangerment finding," at the behest of a US Supreme Court ruling, the EPA is legally obligated under the Clean Air Act (CAA) to regulate greenhouse gases (GHGs), unless Congress acts to remove that obligation, for instance by enacting its own climate legislation.
Is the CAA well structured for regulating GHGs? No. Are the EPA's proposed rules for regulating them under the Act well-designed? They seem to be well thought out, but in trying hard to avoid certain problems stemming form the structure of the CAA, EPA has made them susceptible to potentially successful legal challenges. But none of this changes EPA's legal obligation to promulgate GHG regulations, having found that emissions of GHGs from numerous and diverse sources endanger public health and the environment.