Meh, Diane Black would have probably done worse than Blackburn in a gubernatorial race against Dean. It’s not impossible for them to win statewide, but it’s arguably the hardest state for a Democrat to win nowadays.
The irredeemable pit known as Arkansas says hello.
I'd say it's Idaho. A Democrat hasn't won a Senate election there since the 1974, or a gubernatorial election since 1990, and it hasn't shown any signs of trending left.
The rapid growth in Idaho, particularly with folks from California, could push Idaho left and due to its smaller base population than say Tennessee, it wouldn’t take that much.
Most of the Californians moving to Idaho are conservatives who left CA because they think it's "a socialist hellhole".
So I doubt that will happen.