On the democratic side:
1. Hillary Clinton - has bribed and browbeaten her way to the top of the DNC. She may be a lousy campaigner, but she (and Bubba) is an excellent politician. She knows how to bend the will of people in her way.
2. DNC - not entirely their fault because the Clintons steamrolled the party, but they should have had a stronger backbone. I understand they were worried about a vengeful Clinton machine, but they should have let things play out naturally over the course of last summer - encouraging veteran Dems, and up-and-comers, to make their pitch - and then coalescing behind a preferred candidate. Instead they simple caved to the notion that it was Clinton's time, and kept legitimate contenders away for the sake of party unity. Then, when a crazy old guy almost crashes the party - they lock the doors to keep him out.
3.
@timschochet - no explanation necessary
4.
@squistion - he seems a little young, so he probably does not know any better, but blind loyalty is usually not a good thing
5. Hilbots in general. The inability to see obvious flaws is downright scary.
6. Media - if you did not think the media was in the bag for Clinton, then the clip showing them regurgitating the Clinton talking points following her fainting episode should have ended any doubts. I don't know when it happened, but the media, on both sides - not just the liberal side, has lost all semblance of an independent watchdog. Too many "journalists" are all too happy to spit out the candidates talking points as journalism, rather than taking anything said by either party with a healthy degree of skepticism. They know they only get "access" if they parrot the talking points
On the GOP side:
1. Media - I think the media gets top billing for Trump's nomination. Starting last summer they allowed him to dominate news coverage, giving him loads of free advertising, just because he said something outlandish. It was as if the political reporters had never seen a Jerry Springer show, and felt the need to show it day after day. To Trump's credit - he is the master of creating attention - but again the media is supposed to be above the fray.
2. GOP - GOP had a different problem than the DNC - the GOP allowed all-comers into the tent, and ended up with a three-ring circus. But the GOP failed at the next step which is, after hearing from the contenders - coalescing behind the preferred candidate. Hell, even just forcing out some of the long-shots sooner could have helped a front-runner gain traction on Trump. There was early money behind Jeb, but I don't recall the establishment GOPers rallying behind him, and pushing Jeb to the forefront. Maybe they knew he lacked energy, and ultimately would be a flawed candidate. But, then you had a lot of opportunities to get Paul Ryan and the leadership firmly behind a "mainstream" GOP candidate. At this point, any of the contenders would be in a great position, no matter their weaknesses. Kasich, Rubio, even Cruz. The GOP was simply too disorganized and divided to make this happen.
3. Nascar/WrestleMania crowd - "deplorables" is obviously too harsh. But, this is a voting bloc where critical thinking skills are not necessary. They are easily energized and looking for an outgoing leader - enter Trump. I think they got caught up in the moment, and like the kind of brash bravado that Trump portrays. This is the group that carried Trump to the nomination. Now you have the rest of the GOP struggling to figure out whether to support Trump and a loose GOP agenda, or stay away from the circus. Now there are some supporters who like Trump's attitude, and maybe just hate Obama and/or Clinton, but the bulk of his core support comes from people who are not really thinking through the consequences. (To be fair - I think the Hilbots have grossly overplayed the "Trump is the Devil" card, and while I may not approve of Trump, I don't think the country is going to come to an end if he is elected.)