Sorry, but this election is already over. It's better you accept it now.
Either Trump is your nominee and loses, or Trump isn't your nominee but damages the party brand so badly over the coming months that it can't recover (as if the brand isn't already damaged enough). There are NO other possible scenarios at this point thanks to the media oversaturation of a bonafide STAR like Donald Trump. The public usually isn't paying attention to elections at this point, but because of him they are, and they've already made up their minds.
Better luck next time. Maybe next time don't let your party humor an insane billionaire's racist ramblings about birth certificates, Mexicans, etc for 8 years just because they want to stick it to the (black) Democratic president, and then expect him to just go away quietly when you don't need him anymore.
Just stop.