I want to be clear about this: Nothing about Donald Trump's actions at any point in what passes for his campaign should surprise anyone. Appall, disgust, revolt, sure. In the context of his campaign to date, his apparent refusal to accept the outcome of the election is not outside the margin of calamitous error.
In sense, this is a testament to the durability of the American experiment. Beginning with the election of 1800, interrupted only once (albeit a big once, in 1860), every presidential transition has been accepted by both sides in the contest with no attendant civil unrest of which I am aware. This has led to a justified complacency in the peacefulness of our political process, to such an extent that a major party candidate for President elicits (thus far) the equivalent of nervous chuckles upon his (oh what a giveaway) refusal to say whether he would accept the outcome of an election process in which voter fraud plays infinitesimally less than no part.
The combination of peaceful precedent and buffoonish mouthpiece may lead some to downplay this, but in any other context this would be a disqualifying action. In this election it's just the next news cycle.
Republicans, you own this. Republicans, fix this.