But he does. Earlier ballot challenge cases got dismissed because 1) the plaintiffs could demonstrate a particularized injury, either because they were voters and stood in no different position relative to the population at large or 2) they were marginal candidates or potential electors for such candidate whose claim of injury was deemed speculative.
Trump is in a different position. He could commission a survey in a state that asks the common "Who is your first choice . . who is your second choice? questions. It would then be simple to count the number of instances where Cruz is first and Trump second and translate that into likely lost votes and lost delegates. With that in hand, I think Trump demonstrates the particularized injury the courts say is necessary for standing.
But this is in the end a political calculation, and given widespread belief the courts are very reluctant to interfere to such a degree in presidential elections, Trump will prefer to leave this issue hanging rather than risk losing in court.
Make the above “plaintiffs couldn’t.”
That's entirely speculative. There is no proof that those polled will actually turn out and vote.
Dummett was an opponent on the ballot, so had standing. Case turns on the fact that the Secretary of State has no legal duty to exclude unqualified names from the ballot, but may exclude them if she wishes.