Honestly, if someone started going off on how they're not going to vote for Bernie Sanders, I very much doubt anyone would particularly care, but on the off chance someone did chose to engage, and they were 'of the left,' I very much doubt their impulse would be to try and win that person over.
A more relevant example outside of the hypothetical would be to see what responses someone gets for saying they wont vote for Mamdani, see if the bulk of replies to that are attempts to win that person over. Or like, just how Warren supporters were treated close to Super Tuesday in 2020.
oh, yeah. I've seen this. People get immediately excoriated and insulted. Their motives are assumed to be purely evil.