Good faith and fair dealing are implied covenants of any agreement.
Just to have a little fun with the legalities, is the pledge a contract? It seems a one sided promise by Trump without any corresponding promise of performance by the GOP. Similarly, there is no exchange of consideration. Trump gives the consideration of supporting another nominee and not going independent, but the GOP does not give any consideration in return. If it is a contract, it lacks mutuality. By the same token, there can’t be covenants (such as good faith and fair dealing) where there is no contract in the first place.
Can the pledge be deemed a contract with an implied promise by the GOP to treat Donald fairly providing mutuality and/or the bargained-for consideration? An argument can be made that such a promise is implied by Reince Preibus’ signature on the document. Otherwise, Why is he signing a unilateral promise?