He’s betting that Clinton will have one term then he can run as opposition. If Trump wins Cruz faces another primary fight, against an incumbent.
Not only did you put a shiv in Trump's back by giving the Hildabeast a talking point ('vote your conscience'), you just set yourself to be primaried for your TX Senate seat in 2018, which ensures an establishment RINO or even a 'Rat pickoff.
One thing for sure - you'll never become POTUS.