I think barring an Alabama-type scenario where the discrepancy between Democratic and Republican turnout is immense, 43% or so is his ceiling. A good part of the reason why Bredesen's name rec is (might be? not sure about that poll) so low in that poll is because a significant portion of the people who voted for him 11-15 years ago are now dead. Those people also disproportionately were the ones you needed to win statewide as a Democrat in TN: the proverbial ass-end of the New Deal Coalition.
If turnout (as a share of presidential #s) is like 80% for Dems and 55% for GOP, then yeah, winning is definitely possible.
The path is for the electorate to be around 40% R, 31% D, 28% I. If Bredesen can win 15% of Republicans, win independents by 15-20 points and win 95% of Democrats, that's a win.
That's just not possible. Jones only got 8% of Republicans under extenuating circumstances.