the Dems and the GOP have tried to choose the only candidate who could lose to the other major party's candidate

Hillary is one of the few Democrats who could lose to Trump. There aren't enough "yellow dogs" and sexist supporters for her to win. So, she has to pick up enough people who:
a.) have low standards and can stomach her better than they can stomach him;
b.) have decent standards, but believe the myth that their vote matters or lack the imagination to vote third-party.
It'd be a lot easier for the Dems if Hillary had to step out-- or was encouraged to step out. If so, will it be Biden? 

Trump is perhaps the only Republican who could lose to Hillary. There aren't enough "yellow dogs" and yahoo supporters for him to win. So, he has to pick up enough people who:

a.) have low standards and can stomach him better than they can stomach her;

b.) have decent standards, but believe the myth that their vote matters or lack the imagination to vote third-party.

It'd be a lot easier for the GOP if Trump had to step out, but that's probably not going to happen. So, good luck GOP-- and thanks for playing.