The Lieberman Threat
I've already stated my opinion that Joe Lieberman will have to directly attack the Democratic Party's narrative on National Security if he is to have a chance of winning re-election. But there is an even more pernicious threat to Democrats from Lieberman's candidacy.
New polling says that Lieberman is getting nearly 75% support from Republicans in Connecticut. This means that, in order for Lieberman to win, he will need a large number of Republicans to come to the polls in November and vote for him. Of course, if and when they do that, they will also vote for a lot of other Republicans on the ballot, thus hurting the chances of Lieberman's fellow Democrats in winning back at least three Congressional seats.
In other words, Lieberman on election day will have to run a Republican Get Out The Vote effort.
Lieberman's candidacy is a disaster for Democrats in both messaging and logistics. If Democratic party leaders think they can just whistle past this problem and hope that it doesn't hurt them then they are being either incredibly naive or purposely self-destructive.