The election is not firmly in the hands of the mainstream parties for the first time in my lifetime. The recent decision to eliminate Republican voter input in Colorado was only surprising because it was done in the open.
Both the Democrats and Republicans are trying to ensure that the primary outcomes allow the candidates of choice win the nominations. But Trump and Sanders, both with a lot of popularity, have nothing invested in the traditional structure, no support from the parties, and are still in the game. Both of them also have nothing to lose by pressing on no matter the outcome of the conventions. Game theory would suggest that they both will do that.
If they both make that decision, leaving the Democrat/Republican nominations to Clinton/Cruz, with Trump and Sanders running as independents, it makes it almost certain that the popular vote will be rendered meaningless and the election decided by that anachronism of control, the Electoral College.
There's never been a modern election decided by the Electoral College, they are in some theoretical ways sent as representatives of the States, but they could do as they please. Deals made, ballots counted, and ta-da! Here's a president. The Electoral College exists because the Founders really did not trust the people to decide Presidential elections. It's there as a doomsday fallback for situations just like the one we are working our way into.
It is not possible to predict what they would do with some manner of a four way split in the popular vote. What is easily possible to predict is the aftermath. Pick any one of the four as the winner. What is the response of the other three? Do they fold their tents, roll up their banners and get behind the declared winner as the Constitutionally elected President? Would Hillary do that for Donald? Would Donald do it for Hillary?
What if the Electoral College elects someone other than the one that won the plurality of the popular vote? What if the Electoral College elects someone other than the one that won the plurality of the Electoral College?
And all this plays out in front of a 24/7 media.
Apart from all that, here's my pick for November. It looks like a joke, but they got together the day after they get shut out at the conventions, I think they could win.