Biden, trump and bernie are running for office. If you vote Bernie instead of Biden on principle and trump wins the supposed lesson that Biden would learn is that he needs to be more leftist to get those bernie voters to back him next time (this doesn't really work for individuals but for parties at large. again I am simplifying) So the next election comes up and Biden is supposed to pitch more left to get the Bernie voters and the Biden voters. That is supposedly what happens when people make a third party protest vote.

But what I am suggesting is that there is just as much of a chance for Biden to try to be more centrist, thus lean to the right more, to steal votes away from Trump and allow him to win with or without the Bernie voters.