Are you just talking about the birth certificate thing? That’s really nothing.
Not the birth certificate per se, it’s what the attack betokens. Trump isn’t contrasting issues, he’s going after Cruz as he went after Carson and the others— on a personal level, e.g Cuban.
I expect Cruz will win, but had hope Trump would lose graciously- and thereby, Cruz would pick up his followers. Or alternately, if Trump won, Cruz supporters would flock to Trump.
Don’t see that happening if Trump continues down this path.