As always in such matters it is a useful exercise to replace (R) with (D) and imagine what the Party of Lincoln would say were Trump to be the presumptive Democratic nominee for president. If that were the case I'd cash in the kids' 529s and invest in clutchable pearls and fainting couches.
I truly believe that in a healthy, functioning civil society, we would not have arrived at this point. However, we must play the hand we are dealt (or, more accurately, deal to ourselves as a country), and at this point, the most bitter anti-Hillary, pro-GOP partisan (I think I know someone like that) needs to answer this question:
Is Donald Trump fit to be president?
Or, perhaps more specifically, is Donald Trump more fit to be president than Hillary Clinton?
Personally that does not rise to the level of "legitimate question" but YMMV.
No rush on this; you have until early November.
Oh yeah, almost forgot: given what has come to pass, it is incomprehensible to me how Ted Cruz clearly is so truly, deeply, madly loathed, more than enough to deny him the nomination. You have to work very hard to be so very hated.