As an Alabama resident, I can say that Democrats could foreseeably pick up Alabama's senate seat if Shelby loses to a Tea Party challenger, a distinct possibility. However, even then this Tea Party challenger would have to be exceptionally gaffe-prone (think Todd Akin on steroids) and weak, Hillary would need to have a relatively strong showing in the state, Black turnout would need to be exceptionally high, and the Democrat's nominee would have to be a very conservative, white, and not named Parker Griffith, as well as capable of winning really (atlas) blue parts of the state. Bobby Bright is the only Dem I could think of who could win if Shelby falls in a primary.
Isn't Bobby Bright a Republican now?
Nope. You might be thinking of Parker Griffith.
Actually, Bright flirted with running for state senate as a Republican this year, but opted out. Considering Parker Griffith came back to the fold, Bright running in 2016 wouldn't be out of the question, but he won't win.