with; as I mentioned below, the Bernie Bros. If you're committed to the party issues and getting elected, then you're far more likely to put your personality preferences aside and get behind the united candidate. If nobody has a majority going in, it's up to the delegates to come to an agreement on who it should be. It could be somebody who isn't on the ballot at all right now (though it would be unlikely). You take care of the unity by developing that agreement and making one of the losers the veep.

But....if you aren't a party member or you joined the party to support one person and will take your ball and go home, then don't bitch when consequences happen. IMO, Hillary should have taken Bernie as her veep last time. I don't think she offered. I'm also not convinced that Bernie would have taken it.