Usually, the strongest 3rd-party or independent presidential candidate takes votes from the incumbent party. In 2000, when the president was a Democrat, Ralph Nader got about 3%, helping Bush win. In 1992, when Bush ran for re-election, Ross Perot got about 19%, helping Clinton win. In 1968, when the president was a Democrat, George Wallace got 46 electoral votes, helping Nixon win.
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.