Him not committing to one or the other is going to hurt him either way.
His opponents can now almost easily judge his political assault as a power grab. (since he doesn't really have positions, what more could be deduced) He went from being a Dr. to a senator and won't go back. It's a typical democratic thing to do actually, and if I remember correctly the person most associated with the tactic is LBJ.
Running for president = He's not really a man of Kentucky/ Kentucky is only his stepstone. He's really a federalist and Kentucky is too small for a man with ambitions of being the president.
Running only for senator = afraid to commit because unlike his father he won't be able to stand losing so gracefully only to pop back up in four years.
Not trying to take jabs at him, but putting for what most likely is going to be spun heavily.
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