Actually, the latest numbers for Bredesen if 48% approval overall and 48% disapproval. Even worse, among Democrats his job approval is more like 47%, with 53% approval among Republicans and 45% among independents.
I posted an extensive analysis of the most recent job approval ratings of all 50 Governors at
http://www.democracyfortennessee.org/ - The long and short is that if you rank all Governor's by the job approval ratings among their own party members, Bredesen is tied for 49th. The only governor in the nation with a lower job approval rating amongst his or her own party than Bredesen is Gov. Taft of Ohio who is involved in some kind of ethics indictment.
As for replacing Bredesen from within the party, it's an uphill battle, but it can be done. Just last year an incumbent governor in Missiouri lost his own party's primary... unfortunately, I think the nominee who won the primary then lost the general election (I need to check that).
To win the Democratic primary over Bredesen, he will have to demostrate that he or she can win the general election, will need access to serious money as Bredesen is approaching $6 million in his account, and will need the support of lots of institutional players and party leaders... The last one is easy with everyone Bredesen has angered, and there are a number of very winnable candidates out there (as much as I like Ed, I don't think he's one of them). The real problem is money. Anyone got a few million to chip in?
If on the other hand it is just a disruptive vote of people who just can't vote for Bredesen regardless (like me), then it just has to pull enough votes away from Bredesen in the primary to either weaken him or make him move to the left... or as an independent challenge in the general election to throw the election to the Republican.
There are a lot of Dems I talk to who in hindsight, even though we all voted for Bredesen, agree that we would have been better off with Van. He would have proposed a lot of the same cuts as Bredesen, but as a Republican at least the Dems would have flamed him every step of the way and the state would just be stuck in partisan gridlock. It would have given the Dems someone they can beat up on publicly while rebuilding the party base to put a real Democrat in the Governor's mansion. It's a long-term strategy instead of the must-have-a-democrat-at-all-costs-even-if-he's-evil-like-Bredesen strategy.