Smart man
And why is that smart?
Yes. It would be stupid to endorse either because it would put him in a very awkward position should the other win, not to mention provide the Democrats with ammo for the campaign (i.e. "His/Her fellow Senator does not even think he/she is the right person for the job!").
Having said that, it can almost be seen as a defacto endorsement of Haley since he would be expected to endorse the GOP establishment candidate, which is definitely not Haley.