I don't have to have a rationale argument. I simply have to do the same thing you are doing and that is take a position and question anyone's rational who thinks differently.
Oh I can take multiple different positions as rational. If someone stayed home at the polls, wrote in Sanders, or Cruz, or Mickey Mouse, supported a 3rd party candidate which was their preference, or voted for Hillary I could see a rational argument for that.
Just none for Trump. Trump checks no boxes. Cruz, whom I disagree with his philosophy, and generally think of as having poor character, has some merit to conservative candidates. But voting for an unqualified unvetted non conservative, who flaunts his ignorance so frequently it's become difficult to create a comprehensive list of his most disqualifying characteristics, is definitely irrational.
There's no win there, it's just backing a candidate that was forced through the primary process by the very worst fringes of the Republican Party. (Like the white supremacist delegate he appointed). It's backing team red despite a substantial lack of philosophical agreement, or meritorious achievement.
I suspect you've hired a few people in your life. Imagine looking at Trump's lack of experience, his childish insults, his social media, his fraudulence. Then imagine that during an interview he blew it time after time on basic level of knowledge questions, he makes jokes about someone's menstrual cycle because they asked him a hard question, he mocked another person's disability for the same thing. imagine a job candidate that actively bullies other people interviewing for the job, going as far as insulting their wives. Could you imagine recommending him for a job as a dept head, much less head executive of the entire nation?
If you were looking at this for what it should be, a job interview for the most important position in the country, there's no way you could rationally sign off on Trump.