Your back is against the wall when it is literally against the wall, when they start killing you to silence you. Before then, you have choices.
In 1932, the German left could have unified against the Nazis and outvoted them. But the Communists (KPD) 1932 thought they could outwait Hitler and the Social Democrats (SPD) didn't want to form a coalition with the KPD. The very first antifa, Antifaschistische Aktion, was founded as a unified response to the Nazis. It did not gain enough support to be effective.
Sanders, like any old socialist, is aware of this history. This is why he advocates staking out the strongest possible left position and “Then together, standing united, we will go forward to defeat Donald Trump, the most dangerous President in modern American history and we will fight to elect strong progressives at every level of government from Congress to the school board.”
No, I do not love Joe Biden. No, I do not like his record. Yes, he is still 1,000 times better than Trump. Joe Biden wouldn't have dismantled Obama's epidemic response infrastructure response. Nor would have have kidnapped children on the southern border. Nor defended Nazis as "very fine people."
I don't understand the thinking here. There is a clear lesser, much lesser, evil than the one that is endangering your lives. Why not choose it?