Cruz is the only realistic chance the GOP has at this point
If the Republicans tried to take the nomination away from both Cruz and Trump they would be ignoring the will of a majority of the party and would surely lose. With Cruz, they have a chance and they should take it.
... Cruz has been the only candidate to effectively respond to Trump and is arguably the only one many Trump supporters could live with as an alternative.
The point here is that Cruz, for all his apparent rigidity, is actually a remarkably nimble and capable politician. The assumption that he couldn’t beat Clinton isn’t just belied by the polls, but by the fact that Cruz has shown a mercenary’s willingness to do what it takes to win (and I mean that as a compliment).