It's a pretty popular theory around here that Lieberman is going to jump ship: either he'll caucus with the Republicans, or he'll resign and become Secretary of Defense. While it's certainly a possibility, I don't find the argument convincing.
The "Secretary of Defense" scenario says that the Bush administration will replace Rumsfeld with Lieberman so that the Iraq War can be blamed on the Democrats, while Jodi Rell replaces Lieberman with a Republican. This scenario requires Rumsfeld to resign or Bush to fire him, and while the Publicans are certainly politically expedient when they want to be, I think Bush is just too stubborn to do this, politics be damned. (Rove would do it easily, if he were president, but Bush isn't as facile, I think.)
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Update:Well, shows what I know.)
How about the idea that Joe will switch to the Publicans, or caucus with them as an independent? It's possible, but again, I don't get the sense that Joe is as Machiavellian as that. I think the label "Democrat" might be important to him; he may have run as a CfL but that wasn't by choice. He may not have a lot of loyalty to the Party, but I'll bet it's easier for him to vote against us in all sorts of ways, than it is for him to call himself a Publican or to caucus with them. Heck, even Zell Miller caucused with us all the way through, and he was way wackier than Joe is.
So what if this DOES happen: Joe leaves, and we lose a seat in the Senate (and maybe even control of the Senate)? Well, if it does, then at least we, the netroots, will have the consolation of pointing out to the Democratic Leadership that we had, in fact, called it. The whole "Lieberman will jump ship" theory frankly sounds like paranoia to the rest of the world, but if it comes true, then we might be taken just a little more seriously than we are now, while the DLC will look just a bit more like idiots. Silver lining.