Before Trump, I thought it would have to be something like Rubio/Kasich, Kasich/Rubio. That combo had great potential to win in Florida and Ohio.
Trump changed everything. The only message now is Nationalism. And every GOP candidate that is still running on the old Reagan/Bush/Bush platform and still thinks this is "normal" politics is being left in the dust. As much as I dislike him, Cruz was smart enough to see it coming and has not pinned himself into a Tea Party corner. He's been vague enough on the Nationalist ideal to kind of start agreeing with the rise and garner votes.
Cruz is the only one left that can beat Trump and it really comes down to what people will do once the curtain is drawn. I think lots of people want to vote for Trump and love the message, but are they really willing to pull the lever and admit to their friends 5 years from now they voted for Trump when/if he completely goes off the deep end or admits this was all a hoax. Cruz has positioned himself to be Nationalist Lite with a bunch of religious mumbo-jumbo mixed in.
Obama and the liberal/educated elites constant apologizing for being America has created this. People are fed up. And Trump has unabashedly and crudely gotten up on stage and basically said... "I am proud to be American. I am proud to be a man. I am proud to be white." And he's not apologizing for anything that comes along with that.
The GOP either gets with that message or it will get run over by the Trump train.