I don’t think anyone is seriously denying that Bredesen could win in a massive Democratic tsunami (I have it as Toss-up myself, btw), it’s just a question of whether you believe 2018 will be such an election or not. Unlike the rest of this forum, I’m not convinced it will be, at least not yet.
The rest of the forum isn't really convinced either, the average Atlas prediction shows Republicans keeping the Senate (and therefore winning TN, and no Dem Tsunami).
This assumes Tennessee is the tipping point, when it's quite possible that O'Rourke pulls an upset or ends up doing better than expected against Cruz. Remember Kander ended up being ahead of Feingold after all.