I think the GOP is making a mistake going for Rubio here. They're playing the short game when the long game is more important.
There is a massive chunk of the base (about 35%) right now that is furious with the establishment. We saw bits of this in 2012 but now it is in full effect. This group of people is far, far right and wants things done in Washington that are incredibly unlikely.
If Trump wins, their guy got the nomination, and either A) loses the GE or B) wins the presidency and goes on to not do what they want. Both things are good for the GOP. In scenario A, they say, "See? Should have picked the establishment guy." Scenario B is the same way. In the end, this group's rage may subside and they head back to the party.
If Rubio wins, there is no possible way he can live up to what they want. Next election, the rage against the establishment is worse and the party may head toward a split.