Except, of course, that any vote not for the Democrat is a vote for Trump. That's what happened in 2016.
Actually no. The third party vote helped Hillary. According to CNN exit polling when third party voters were asked whom they would have voted for in a strictly two candidate race, no third party candidates. 19% answered Trump, 16% Clinton. 65% responded they wouldn't have voted.
In other words with no third party candidates considering 9 million people voted third party, Trump would have added 1.7 million more votes than he received, Hillary an additional 1.4 million. If no third party candidates, 5.9 million who those who voted wouldn't have. Such was the disdain for both major party candidates that officially registering their vote against both Trump and Clinton drove them to the polls.https://www.cnn.com/election/2016/results/exit-polls
Hillary was actually the beneficiary of the third party vote. Third party voting usually hurts the Republican more than the Democrat.
Having voted third party I've been told many times by Clinton supporters my vote for a third party candidate was a vote for Trump. I also have been told by Trump supporters my vote a third party candidate as in reality a vote for Clinton. It was against both. Neither Trump nor Clinton seen their vote tally go up by one once my vote was counted. It was a neither vote which helped neither major party candidate.
Hence, it is vital the Democrats come up with a candidate attrictive to independents. In 2016 3% of Democrats voted third party, 4% of Republicans voted third party, 12% of independents voted third party. Time to learn from the mistakes of 2016 when it comes to nominating a candidate. Candidates matter.