a decent number of bernie supporters will write him in, vote third party, or sit out. but honestly, the democrat and left leaning independents do a better job of holding their nose and voting for the party's candidate than republicans typically do.
i said it before and a couple of the conservatives laughed at it, but mitt romney would clean up this november against hillary. the issue is the republicans instead are going to nominate the one person in this election that is more repulsive to moderate/independent voters than hillary in trump, or they're going to alienate a huge chunk of their party and nominate someone else and the voters are going to give that nominee a big f.u. in november letting hillary coast into the white house.