Cruz and Trump don't attack each other. Cruz is at second place. Trump and Cruz met and discussed things a long time back. If I were Donald Trump and I wanted to support Ted Cruz, what better way than to destroy all the other candidates with withering criticism? "Bush is low energy". "Would you vote for a face like that?". "Marco Rubio's a lightweight". "Christian Scientist? What is that?". He says things that not even the most evil attack PACs would about the candidates in the exact right way to sow doubt about that candidate's fitness to be President.
And at every turn, Trump says something so radical that he's preparing the Republican establishment to back anyone, no matter how "ultra-conservative" that guy might otherwise seem. His numbers keep spiking, but he seems to be trying to find the thing that he can say that will go so far over the top that his support will fade. But he doesn't want to do it too soon.
If he really is just supporting Cruz, then look for something to happen after Christmas, during the runup to Iowa. Maybe Trump will say something like, "You know, Hitler wasn't bad for Germany. Maybe democracy is overrated. Maybe the only way to fix things is to ban the Democratic Party and start over." The ensuing scandal forces him to drop his bid. Who will his supporters go to? Lightweight Rubio? Crazy cultist Ben Carson? Carly Dogface? Dead Battery Bush? No. They'll take up Cruz's banner almost immediately.
If I were a billionaire, why would I want to be President, after all? Why not just do what all the other billionaires have done since the inception of the Republic and find a candidate I support and bankroll him?
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*