Trump's route to the nomination is not as direct as the media is making it sound. The party machinery doesn't like him, both on personal grounds and the belief that, in the end, he is unelectable. He is leading Cruz and Rubio now, about 34% to 19% to 13%. However the 34% not yet in one of these corners are not likely to break well for Trump.
Cruz will likely get the 4% of the support currently registering for Paul and Huckabee bringing him to 23%. Rubio will likely get the 11% of support currently registering for Bush, Christie and Kasich bringing him to 24%. The 14% currently supporting Carson and Fiorina are more difficult to assign as the field narrows. Also, the 5% currently undecided are not likely to break to Trump and probably not to Cruz.
Therefore, absent party leaders making overtly anti-Trump rules for the nominating process, it appears inevitable that we will go into the convention with no candidate possessing a majority of delegates. As delegates are released from their commitments in the second ballot and beyond, we can expect a general move away from Trump.
It is very unlikely, but it is possible that Trump could grab the nomination, especially if he made a deal with Cruz. Current polls, however, give him little chance in a general election. This is especially true because turnout among Trump supporters is likely to be weak. In other words, though the polls currently show him just 1.8% behind Clinton and 2.0% behind Sanders, his actual ballot box deficit is probably much greater. In essence, Republican leadership, probably correctly, view him as unelectable.
While the nomination of either Trump or Sanders is unlikely, the probability that they both get nominated is non-zero. What happens when two 'unelectable' candidates are nominated? Well, I don't know for sure. I am pretty sure that when the campaigns go negative, they will both hit home with regularity and, by the end, the general mood of the electorate will be one of disgust. I would expect that we would see the lowest turnout in history.
It is a nightmare for both parties. The ensuing race to the bottom will besmirch both parties and the U.S. in general. I suspect that much of Western Europe would view it as a horrific spectacle as a boorish populist savaged a good and reflective man.
I am simply taking solace in the probability that neither will happen. Hillary Clinton, as damaged as she is a candidate, will almost surely limp to the nomination. The Republican party will undoubtedly do whatever is necessary, including manipulating the primary and convention rules to assure that Trump is not the candidate. That turns it into contest between the Tea Party beknighted Ted Cruz and the final candidate of the moderate Republicans. The current assumption is that the party leadership will rather unenthusiastically give the nod to Rubio.
The moderate wing has historically gotten its way from Bush to Dole to Bush to McCain to Romney. However, we must acknowledge the strength of Cruz, especially if Trump delegates as they abandon him go strongly to Cruz.