If it's true that Joe Lieberman's announcing his retirement tomorrow it's a smart decision: at this point the only voters who like him are Republicans and that's not going to take him very far in a heavily Democratic state.
A poll we did in Connecticut in late October found that his approval rating was 33% with 54% of voters disapproving of him. Lieberman's problem if he had decided to run for reelection at this point is that he really has no remaining base. He has significant liabilities with Democrats, Republicans, and independents alike that would make the path to another term very difficult:
-Republicans. Lieberman does maintain positive approval ratings with Republicans at a 48/43 spread and those are the voters who gave him his highest level of support in 2006, more so than independents. But that 48% approval is well below the 70% of their votes he got the last time around and a Tea Party infused GOP would have been a lot less inclined to vote for Lieberman in 2012 than its voters were the last time around. A poll we did in early October found that even if Republicans nominated a weak candidate- Peter Schiff who finished third in the 2010 primary behind a candidate who dropped out- Lieberman's share of their vote would drop to 24%, barely a third of what he got the last time around.
-Independents. Lieberman won a majority of the independent vote in 2006. That's a really hard feat- even if they're called independents most independent voters either usually support Democrats or usually support Republicans. Charlie Crist learned that the hard way last year, getting only 38% of the indy vote in his Florida Senate campaign. Lieberman has fallen way out of favor with independent voters over the last five years though and there's pretty much no way he could have won a majority of their support again. His approval with them on our last poll wasn't much better than it was with Democrats- 34% giving him good marks, 49% bad ones. When we were testing three way match ups in early October Lieberman finished a distant third with independent voters.
-Democrats. Most Connecticut voters are Democrats and they're the folks who like Lieberman the least. Even after he lost the party nomination in 2006 he still won a third of his former party's vote. But his approval spread with them now is 24/64 and he was polling at 14-16% with them in the three way matches we tested in early October, less than half the support he got from them the last time around. Endorsing John McCain may have been a final straw for a lot of the Democrats who stuck with Lieberman even after his loss to Ned Lamont.
You have to give Connecticut Senators credit for this: they can read the writing on the wall. When it became clear Chris Dodd was not going to be able to survive his poor approval numbers he got out and it looks like Lieberman's doing the same thing.