There is no way to know why Trump didn't run in 2012 except by asking him. Why would his supporters know the answer to a question like that?
Premising the hypothetical question with a biased statement about both Romney and Trump is disingenuous.
A more appropriate question to ask is, why didn't Romney win an election that should have been a cakewalk?
Ifa, woulda, coulda, shoulda.....
Asking a negative hypothetical question to prove something that didn’t happen, is a non-sequitur.
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What you said! :-D