His continued support of the Iraq War and attacks on the patriotism of its opponents are the main reasons for opposing Joe Lieberman, but his history of attacks on fellow Democrats and liberal ideals is much longer. There was his support of DOMA in 1996, his scolding of Clinton during the impeachment hearings in 1998, his flirtation with private accounts during the early days of the Social Security fight, and his cloture vote on Alito. While his overall voting record isn't bad, it's hard to see Ned Lamont as anything but an upgrade, especially as far as winning media battles is concerned.
What's really exciting about this race is the message it'll send to safe-state Democrats who may occasionally be driven to Liebermania by the siren song of the anti-partisan media: bashing your party for personal gain is unacceptable, and Democrats in safe seats are expected to do their part in moving the country leftward. Betrayals of the party and liberal ideals, whether for reasons pragmatic or psychological, may come with the consequence of a primary defeat.
I generally agree with folks like Petey who think that moderate-seeming Democrats will attract more votes, but Joe Lieberman is the kind of moderate who gives his party a radical image. Garance has a good post on this. Criticizing "extremists" in your party and making opponents of the war look like unpatriotic radicals does nothing to help Claire McCaskill and Harold Ford win their Senate races. By painting a picture of unpatriotic extreme antiwar Democrats, Lieberman damages the party's brand and hurts Democrats everywhere.
Triangulation makes sense as a strategy for individual candidates, but it's not a strategy that an entire party can engage in. In a country with an established two-party system, the media will define the space of moderate opinion relative to the parties themselves. No party will be able to gain a lasting reputation for moderation by compromising and moving towards its opponents. All that'll happen is that the space of moderation will be narrowed, and opinions that previously were considered moderate will be regarded as extreme.
Consider the idea of invading Iraq. Even setting aside the WMD issue, it's hard to imagine that early poll numbers in favor of invading would've been high if we were under a responsible Republican administration that itself rejected the idea of invasion as ridiculous. Support for the war would then fall outside the range of acceptable moderate opinion. This analysis applies better to new issues where minds aren't made up than old ones where most people have come to a firm opinion, but on things like foreign policy proposals and judicial nominations, we need to realize that the battle of public opinion is still out there to be won. Lieberman must not be allowed to sabotage Democrats by narrowing the space of moderation so that our views look extreme.
I don't want my safe-seat Democrats triangulating into moderate positions. I want them to explore new territory on the left, so that when our Arkansas and Nebraska Senators triangulate off of them, they end up in positions that are fairly good, or at least non-destructive. And that's why I have no use for Joe Lieberman. Where Lamont would stretch the field leftward as a moderate personality with progressive views, Lieberman compresses it and perpetuates negative stereotypes of Democrats. It's time to remove him from politics, and threaten anyone who follows his path with a similar fate.