Oh, must he? The longer Trump is in race taking potential support from him, the better for Cruz? Makes sense. Funny how no one called on Trump to support Cruz in Iowa.
what the real danger is:
party regulars will infiltrate the Cruz & Trump delegations. People who are Trump first, Rubio 2nd choice will become Trump delegates.
People who are Cruz first, Rubio 2nd choice will become Cruz delegates.
at the multi-ballot convention, it will prove to be impossible for Cruz or Trump to gain a majority. Even if they work out a deal.
Not saying it is guaranteed, but it is a danger. I am not sure Cruz has the organizational resources to monitor delegate selection in all 50 states. And he cannot afford to purge the Rubio 2nd choice people from his ranks.