Thanks to
Kevin_Kennedy for starting this thread.
I've also noted Rand Paul's entrance into the race here:
The 2016 IN and OUT thread. US Message Board - Political Discussion Forum
I'm writing that not because I would like responses. Actually, the goal of the thread is just to log when candidates jump in and then out, for statistical purposes, so as a form of "log", I think that you all will find it very helpful with time.
It should also be noted that Sen. Paul is simultaneously mounting a Senate re-election campaign for the Commonwealth of Kentucky. That's not totally unusual: in 2000, Vice-Presidential candidate Joe Lieberman (D-CT) also ran for re-election to the Senate that year, and won.