Cruz trumps Trump on constitutional law
The allegations thus far are pretty nebulous, because as soon as they become specific they'll be shot down faster than John McCain.
Trump's just trying to raise uncertainty about a Cruz candidacy...McCain, I think, is struggling to retain a voice on the national stage.
Having said that, sure, it's possible that should Cruz win the White House, someone may decide to challenge his legitimacy in court. But presidents face legal challenges all the time, and I doubt that a Cruz administration would shy away from swinging a legal sledgehammer when it had to.