I don’t see where Trump has any standing to do this.
Trump has standing because he is running against Cruz on the same ballot.
RE: I don’t see where Trump has any standing to do this.
The term “Standing” means for the ability of a party to demonstrate to the court sufficient connection to and harm from the law or action challenged to support that party’s participation in the case.
Trump should be able to demonstrate that his elections and vote can be affected by a person who is not even eligible to run for President.
That is the problem with this whole issue - who has standing? Cruz can't do it - who would he sue? A voter can't do it - no standing. Perhaps the only one who can do it is another candidate. But then you get to the question of whether the court will hear the matter even if the plaintiff has standing if they do not consider the issue "ripe". They may decide that no injury has occurred unless and until Cruz wins the nomination - if he doesn't win, the issue is moot. The court does not pre-emptively rule on POTENTIAL injuries, unless the likelihood that the injury cannot be remedied afterward is extremely high.
Ironically, a court might find Trump to lack standing because he'd likely win, regardless of who else is on the ballot.