Sure, the Republican primary electorate found Trump compelling, and that's why they won. And that's what scares me. I don't think they necessarily should have found any of the others compelling (what an awful field of candidates, I mean, seriously, what happened there?) but that's beside the point.
In 2000, Bush mainly kept his mouth shut. But Gore's campaign manager made a series of public statements along the lines of "Gore has clearly won, and if this goes to Bush it's a travesty of the election." That didn't help heal or unite the country after the election; arguably, the division has remained ever since.
Trump could do the same thing, but do it much more loudly, bombastically, and in a more attention-grabbing way. The only prevention would be for him to lose so strongly that nobody listens, and everyone writes it off as the whining of a bitter self-deluded loser.
Trump didn't attract enthusiastic supporters across party lines because they were apathetic jokesters. He may sound insane to you, but to a very large "silent majority" he is the first voice of sanity in a long time.
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If God descended from heaven and snapped his fingers and declared, "This November, you shall decide between Donald Trump and Hillary Clinton," then I wouldn't find it particularly surprising or worrying that ~50% of the electorate would support Trump.
But the fact that people chose him out of a wide field during the primaries is a totally different story. Yes, none of the other choices were particularly compelling, but neither were they the inconceivable choice represented by Clinton.
It's not that they believe in him more than Clinton, it's that they believe in him more than Rubio and Cruz and Christie and Jeb Bush.
A big chunk of voters basically just said "fuck it" to the whole process and gave up on even the pretense of sane politicians. Who are they going to choose for Congress? Who are they going to nominate for president the next time around? Maybe they'll realize this was a mistake and get back to some form of sanity, but I'm not optimistic.