Jefferson Davis was actually opposed to succession. So what if he became a Southern Unionist?
Not much would change I imagine, except that maybe there's a chance he gets picked as Lincoln's VP in 1864 rather then Johnson; that'd be interesting.
Tennessee was much closer to the union than Mississippi.
Anyway, I'm not sure if Davis would be a War Democrat in this issue.
True, but Johnson became famous for being the only Senator from a seceded state to stay with the Union, if he's not the only one, he may not receive the same fame he did OTL, I am unsure through.
Andrew Johnson also gained fame by being the effective military governor of Tennessee from 1862 to 1865, which Lincoln was reportedly impressed by.
Would Johnson even be appointed Military Governor?
He was the most prominent Southern Unionist from Tennessee OTL, if he's still around, Lincoln might still appoint him.
What would be necessary to have Davis stay with the Union? For the Republicans to abandon all their principles and embrace the Crittenden "Compromise." Davis was not a "secessionist per se" like Rhett or Ruffin who were just looking for some excuse to break up the Union; he was sincere when he spoke of his love for the Union; but it would only be a Union that would respect his idea of Southern Rights, and this was just not possible after Lincoln's election.
All of this requires a significant change of personality for Jefferson Davis in the years leading up to the war. He was never a Fire-Eater like Robert Rhett or William Yancey, but he was a strong defender of the South's constitutional right to maintain slavery and defended the right of secession.