Hypothetical primary among Republicans with Trump and Cruz as the only options:
Between Trump and Rubio:
Previous data show Trump easily beating ¡Jabe! and Carson by even wider margins. And this is among Republicans--in states with open primaries, Trump's numbers will improve further still (Texas, where Cruz has a home field advantage, is an one of those open primaries).
The prospect of the field being winnowed down to Trump and a single non-Trumpian candidate no longer is enough to get the establishment there. While Trump has dominated for several months, only in the last couple of weeks has crossed the 50% threshold against the entire field. When Reuters began polling on the mano a mano match ups in mid-December, both Rubio and Cruz were beating Trump. That is no longer the case. Trump's support isn't leveling off, let alone attenuating. To the contrary, it's still growing.
The nomination is now genuinely Trump's to lose. The first week of February will see a media manufactured dethroning when Cruz probably places first in Iowa, but the faux narrative will take a one-two punch in New Hampshire and South Carolina when Trump wins both states by substantial margins. Nevada, another state Trump will win, is just a few days later. Then the results will start pouring in rapidly and Trump's national advantage will really begin to manifest itself in the delegate counts.
This presumes no successful malfeasance by party operatives, of course. There will be future attempts, but Trump has almost effortlessly frustrated the ones that have been tried so far, and the smart money says that he'll continue to do the same going forward.
For your viewing pleasure, Trump reframing protester activity at a recent rally (jump to the 25m44s mark):