Of course, both Democrats say they'd like to tackle both climate change and health care and revamp immigration and eliminate poverty and enact labor-law reform and overhaul education... and a million other things. But, realistically, a Democratic president in 2009 might only have the opportunity to do one or two "big" things in the domestic sphere, aside from the budget—especially if Iraq and the flagging economy suck up most of their time and attention—before they start to lose political capital and momentum. So the question of what, exactly, the candidates want to prioritize isn't trivial.
I don't know what the "best" answer here is (from a liberal standpoint, that is). Some climate scientists say we have less than a decade to start slashing carbon emissions in the United States if we want a shot at avoiding the worst effects of global warming (we'd also need to figure out how to get China and India to start curbing their emissions—and pronto). That's a good contender for top of the to-do list. But health care costs and access are pressing issues, too, and universal coverage is the kind of thing that could help cement a Democratic majority for years to come. Plus, universal health care might be easier to pass during an economic slump than a cap-and-trade bill that raises energy prices—even if there are ways to cushion the pain.
Anyway, a lot of observers have noted that Bill Clinton probably pursued his domestic policy agenda in the wrong order: i.e., From a tactical standpoint, he should've done welfare reform before health care, and he shouldn't have alienated organized labor with NAFTA right before he needed union support for his health care bill. I haven't seen anyone argue about what order the next president—Democrat or Republican—should try to do things, and, admittedly, it gets tricky when Iraq and the economy are tossed in to the mix.
Of course, asking a Democrat which they want to do more, deal with climate change or provide universal health-care is like asking a mother which child she likes best.
My instinct would be that the most overwhelmingly pressing issue is climate change, because that is a matter of preventing global catastrophe.
(posted by Ewan)