The recent spate of portents of economic downturn are right in Clinton's wheelhouse as someone who is always prepared with a stack of "what I'm gonna do for you" proposals to nearly every demographic group.
A worsening economy should help both Clinton and Edwards partly because it makes people worry about more bread-and-butter issues that go right to their policy-centric electoral approaches. But I think it also favors them in terms of tone. When you're down, whether it's your own fault or you're a victim of circumstance, there is a human tendency to want to hear someone railing against the system.
Obama's rhetoric may do that, but Clinton and Edwards offer an indignation that is more soothing in the short term.
Update: Obama has reeled off a couple good answers, including one on how he would raise taxes without hitting the middle class and retirees. He included some good examples that people could relate to (like Warren Buffett paying a lower tax rate than his secretary) mixed with some specific policy relief proposals.