Regardless of who it is, the one sure way the Dem nominee will lose is by advocating repealing all of Bush's tax cuts. The Republicans will appeal to the middle class by stating that the nominee wants to raise their taxes. The Democratic party will also be tarred as a party of tax raisers.
The way to handle the issue is to advocate repealing the tax cuts for the wealthy, but keeping the tax cuts for the middle class. That way the Republicans can't claim that the Dems will raise taxes on the middle class, and they will be forced to defend tax cuts for the wealthy. This is sort of a mirror of Clinton's '92 strategy, when he advocated raising taxes on the wealthy only.
Of the main candidates, only Dean and Gephardt don't seem to get this. Gephardt wants to repeal all the tax cuts to pay for his health insurance plan. While I might agree with this on policy grounds, it is political suicide. Dean apparently is concerned about the budget deficit. To call for repealing middle class tax cuts in order to balance the budget is not only political suicide, it is incredibly stupid. It will turn the party of FDR into the party of Herbert Hoover.