I had the same "soul searching" moment about a month ago, asking myself just how partisan I am, and I determined that if Trump were the D nominee, I would absolutely not vote for him. But, more importantly...
Donald Trump could not have won a Democratic primary given all of the racist, sexist, homophobic, xenophobic, Islamophobic, etc. crapola he's spouted. But, to indulge the OP with the hypotheticals, in this scenario I would pull a jfern and be #ReadyForJillStein.
... this. One of the reasons I AM a Democrat is because someone like Trump would not find the party welcoming.