Yup, the GOP could have shut him out and let him run as an independent or democrat.
I remember how the GOP was terrified of exactly that; seeing it as a 1992 redux, with Trump as Perot - another wealthy businessman jumping into the ring to dilute the 'conservative' vote.
And how happy the GOP was to have *forced* Trump to campaign as a Republican... and bound to support the GOP nominee for President.
I saw all the posts at the conservative message forums praising the GOP's political acumen, too.
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Face it, the only reason we are in this situation is due to a significant plurality of GOP primary voter-base consisting of revenge-driven nihilists seeking to 'destroy the GOP' and wanting America to 'suffer' for electing 'the Kenyan monkey'.
Until those are replaced, they *ARE* the face of conservatism and will repel everyone not like them.