Joe Lieberman, for obvious reasons, should be progressive Democrats' first choice for a primary challenge. But let's not stop there.
California's Dianne Feinstein has been an inconsistent Democrat for years. Her recent flirtation with the idea of a comprimise with Republicans over Social Security should be the last straw for a truly progressive candidate to step up and challenge her in '06.
The liberal legislature in CA contains several truly progressive rising stars, one of would surely respond to a demand from the blogosphere to challenge this weak-kneed centrist. Concerns about losing the seat altogether are valid but remember that dKos favorite Barbera Boxer won in a landslide in 2004. Short of Arnold, who seems unlikely to pursue a seat in the Senate, (more the executive type) the CA GOP has a miserably short bench.
A more united Democratic caucus should be a long-term goal of the Party, and California is a state that should have two real progressives representing it in the Senate.