I think one needs to sympathize with John Dingell's view of the climate change issue. Environmental leaders have concluded that it's impossible, in the short term, to pass legislation adequate to the scale of the problem. The inconvenient truth, it seems, is just a little too inconvenient and people don't want the kind of taxes and so forth that would put a dent in emissions. So, instead, they've hit upon the most convenient solution: Higher CAFE standards, easy, popular, and with the costs hidden and mostly born by car companies.
This is, however, a bit inconvenient for the auto industry and those who work in it -- i.e., Dingell's constituents. What he's saying with his "yes to unrealistically aggressive action on climate change, no to high CAFE standards position" is, basically, "stop giving me shit about this." And he's right. Either Democrats are going to commit to taking politically inconvenient -- inconvenient for all of them -- stands to curb carbon emissions, or else emissions won't be curbed. At the moment, the party isn't there. So why should he be singled out as a figure of opprobrium for doing, in essence, the exact same thing as everyone else and refusing to take a stand that's inconvenient. Your average Democrat who wants to raise CAFE standards isn't running any political risks by doing so.