Apparently it was when they let Sanders join the Democratic party in order to run for president.
But I don't think there was any other choice. They needed someone to make the primary appear to be a contest and everyone knew that Clinton was the Democratic party's choice. So no other savvy Democrat wanted to even run.
I think that they picked Sanders because they knew he wouldn't win, but he could energize a new batch of voters. Unfortunately, he didn't. He energized a batch of nonvoters who then stayed home in droves.
Tom
Nobody "let" me join the Democratic Party. I just picked a box on my voter registration form, and suddenly I was "joined." There have also been crossover politicians - Republicans who were former Democrats and vice versa. If I'm not mistaken, I think Trump was once a Democrat. Would either party have the authority to not let someone join if they wanted to join?
One can just as easily blame the independent and third party candidates, especially in states where neither Trump nor Clinton got an absolute majority. Trump won states like Michigan, Pennsylvania, and Florida with very slim margins - just a few thousand votes.
And the idea that "everyone knew that Clinton was the Democratic Party's choice" - there's something fundamentally wrong with that, especially if it's decided before the primaries, when the party membership supposedly has the opportunity to make their own choices.