If Corker had any sense, he'd endorse Bredesen. I mean, he's leaving in January. What does he have to lose?!
It wouldn't make any difference at this point. Blackburn, it seems, was always destined to win. The historical pattern of repeat candidates failing with their comebacks seems to be repeating itself, as it did with Feingold, Bayh, and Strickland back in 2016.
Bayh and Strickland suck as examples in this, especially Bayh, who was a deeply flawed candidate due to his post-politics work
(if you saw the 2016 coverage of his candidacy, you should know). Both those states were definitely, and continue to be, winnable for Democrats, but if Bredie loses, it will not be because he turned into a terrible person/candidate after leaving office but rather because Tennessee has simply become too Republican-leaning to win, even in a very favorable cycle.