First off Trump is not Goldwater... Goldwater was an extreme right wing conservative much like Ted Cruz... Trump is a populist... His support comes from a much broader base... Much different situation...Many people that say they won't vote for him when polled but will do so in the election booth... I could be wrong but I don't believe the polls... It's just a hunch and I could be wrong but I don't think they reflect the actual situation on the ground... I will stick with my Trump idol... The thought of not doing anything possible to keep the Clinton's out would really bother me... It's something like writing in Ted Cruz on the write in paper ballot... If Hillary wins Hillary wins... It won't be with my help.
On the other hand, there could easily be a significant number of Republicans that are saying publicly they will vote for Donald Trump in the General Election but decide to do something differently when it comes to the actual vote.
From my own experience, many Republicans are trying to convince me to vote for Trump because of their legitimate concern of Hillary Clinton back in the White House. While I will publicly and privately hold firm in my decision not to vote for Trump ( barring something unexpected, I'm writing in Ted Cruz ), many others don't want to be looked upon as going against the one remaining current candidate that could potentially beat Hillary Clinton. But what they do in the voting booth ( assume they even actually vote ) could be something entirely different.