(2) For any manner of binding or allocating delegates under these rules, if a delegate (i) casts a vote for a presidential candidate at the national convention inconsistent with the delegate’s obligation under state law or state party rule, (ii) nominates or demonstrates support under Rule No. 40 for a presidential candidate other than the one to whom the delegate is bound or allocated under state law or state party rule, or (iii) fails in some other way to carry out the delegate’s affirmative duty under state law or state party rule to cast a vote at the national convention for a particular presidential candidate, the delegate shall be deemed to have concurrently resigned as a delegate and the delegate’s improper vote or nomination shall be null and void. Thereafter the secretary of the convention shall record the delegate’s vote or nomination in accordance with the delegate’s obligation under state law or state party rule. This subsection does not apply to delegates who are bound to a candidate who has withdrawn his or her candidacy, suspended or terminated his or her campaign, or publicly released his or her delegates.
The only way is for Cruz to drop out, and then the states Cruz won conspire with the GOPe to support a white knight, as only Cruz has enough wins to reach the 8 state threshold of free delegates.
-PJ
I missed that one - so at least on the first ballot, only Cruz and Trump. But after the first ballot a lot of delegates become unbound, so it would be possible at that point if they reopen the nominations.