Trump has a Facebook post accusing Cruz of telling many lies about him. The key passage in it: “One of the ways I can fight back is to bring a lawsuit against him relative to the fact that he was born in Canada and therefore cannot be President. If he doesn’t take down his false ads and retract his lies, I will do so immediately. Additionally, the RNC should intervene and if they don’t they are in default of their pledge to me.”
This tells you something about his view of the Constitution, and his view of the RNC. The constitutional argument he is making against Cruz is one that he is willing to set aside if Cruz does what he wants; he doesn’t care about it independent of its use to him. The RNC is being unfair to him if it refuses to “intervene” against other candidates in the race when they say things he considers lies. Here, too, there’s a threat: that he will make a third-party run.
The RNC does not referee every fight in a Republican primary over which campaign charges count as lies. If it did, it would be a very busy institution. But Trump is all about disrupting the old rules. He wants a new RNC: a party establishment that will step in to protect him from his opponents.