Franklin Foer posted a column at The New Republic (reposted at CBS News) arguing that Edward's only chance to win is to go negative on Kerry.
http://www.cbsnews.com/stories/2004/02/18/opinion/main600986.shtml
The upside is that he still has an outside chance of bringing Kerry down. The downside is that the more bark he peels off of Kerry, the less likely Kerry will select him as the VP candidate.