2016 -— democrat in office. Bernie Sanders takes away votes from Clinton. Trump wins.
The democrats are in a tough spot. Do they go full on progressive and lose the traditional democrat vote, or do they stay more traditional, and lose the young progressive vote? If they had more time, they could woo both groups, but they can’t get that done in the next several months.
Sanders probably didn’t run as an independent or 3rd-party candidate because about half of the states have sore loser laws that would have prevented him from doing that, since he aleady ran for president, in the democrat primaries, that year.