Although a few diehards are burning their voter ID cards(how quaint) most seem to be falling in line with the "presumptive nominee".
But there could be some shenanigans at the convention.
There has even been some talk of running a third party candidate to deny Trump any chance of winning but to help the down ticket candidates in what is generally believed will be a blowout. The problem with this is that it would be incredibly expensive and there is no candidate available who could capture the hearts and minds of voters(assuming Republican voters have hearts and minds in the first place).
Some have even posited that Trump will successfully use the same tactics against Clinton that he used to destroy the Republican field. This also seems unlikely to me and that the harder he tries to insult and belittle Clinton the worse he's gonna look to the general electorate. The RCP average has Clinton 6.5 points ahead at this point and I suspect that even the Bernie Bros will come to their senses once they are faced with a real possibility of a Trump presidency.
