If you're a Wisconsin resident or know people in Wisconsin who support Dean, email or call Senator Russ Feingold and ask him to support Dean, or even write a letter if you have time or think it will reach him or his staff in time. Contact info is below.
Email: Russ_Feingold [at] feingold.senate.gov
Again, sorry if you've seen it before.
PS - If you're from a state coming up, or even a month away, consider doing the same. Even if they don't endorse him right away, they might think about it again if he wins Wisconsin. If they did endorse him right away, well, it would be some extra good press.