The democrats understand what many people here on FR do not. You do not dilute your support by involving third party involvement. In this case, they did it for the Republicans by getting Robert Sarvis in the race. Remember Ross Perot?
A bit of history from an old timer. It is always much sweeter to elect your own candidate than theirs. There is an old saying in politics, which ironically I picked up in Virginia in the 60’s. “He is a son of a witch, but he’s our son of a witch.” That applies perfectly to the Governor elect in Virginia, but he’s theirs, not ours. Apologies to Witches.
It’s right out of the Clinton playbook - split the vote and get elected with less than 50% of the vote.