It's worth noting that on a legislative time scale, cap-and-trade was really very, very, new, and most important laws tend to take a long time, often spanning several Congresses, to pass. Looking back...in 1992, global warming was a two-sentence throwaway at the end of the Democratic Party platform. If anything, it's even less prominent in 1996. In 2000, the Al Gore platform contained a fair amount of climate rhetoric -- but proposed almost no specific actions. No change in 2004: plenty of rhetoric, but only vague calls to action, mostly focusing on international agreements. No cap-and-trade, no carbon tax. So for Democrats, the idea that Congressional action is needed to limit carbon emissions as a core party principle only goes back to the 2008 campaign.
(Image: The Daily What suggests an novel solution to the crisis.)