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Trump cannot get the GOP nomination, because none of those not presently supporting him would ever consider voting for him.
That is not so for Cruz. He is the candidate most likely to pick up the votes of those falling out of the struggle.
Cruz will be the last opponent to Bush left standing.
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Bush will get dumped and then Rubio will unify the RINO vote.
The Establishment plan is to use Carson and Cruz to pick off naive conservatives seeking a vanity candidate in order to defeat Trump.
If Carson drops out, most of his support would go to Trump. Donald is both the #1 first choice and the #1 second choice.
http://i2.cdn.turner.com/cnn/2015/images/08/17/rel8a.-.gop.2016.pdf
Trump is the person the Establishment wants to defeat the very most. The RINOs will sacrifice anything to stop him.