How so? Cruz owes Trump less than nothing. If Trump is nominated, he is no more likely to campaign for Trump than Reagan campaigned for Jerry Ford back in 1976. You probably don’t recall how bitter Reagan was about that loss, And while you are looking, please notice that Rubio says he will not accept the VP slot under Trump. Both men are young and ambitious.
I actually think Reagan was out of line when he campaigned against Jerry Ford because Ford was an incumbent president. Sure, Nixon appointed him veep and he inherited the office, but one generally sticks with the incumbent if he is running. Besides, after the Watergate debacle, it was probably just as well that Jerry lost to Jimmuh. It made Reagan look that much better 4 years later.
Cruz seems to be still beating the "contested convention" drum. No good will come of that. Just makes him look like he hungers for the office too much.