Trump has a plurality of support with the majority of republicans backing the other candidates. Sen. Cruz has made an appeal for those people to abandon those candidates and unite behind him. Basically if you feel "anyone-but-Trump" then Cruz is your man.
So let's say that Cruz takes on all the supporters of Rubio, Kasich, and of the candidates who have already dropped out. Cruz is poised to beat Trump at the party convention. Despite his earlier promise to the contrary, I think that Trump would make a run as an independent candidate. He'd see it as a stab in the back and would not have any reason to keep his word.
Which party's supporters are going to be siphoned off by Trump running as independent? Uh huh...
Pass the popcorn, Hillary.