So left-leaning voters can’t ask for more? It’s by design. At some point the table must be overturned.
The thing to ask for is ranked choice voting.
Why not now?
You answered your own question already:
If he does get on the ballot as a spoiler, I won’t vote for him.
You said it yourself. In the current scenario this version of asking for more only makes the road easier for R.
Look what happened to Bernie.
I registered Democrat after decades of being registered Independent specifically to vote for Bernie in the primaries. I feel he was shafted by the DNC both times, just ever so slightly more subtly the second time.
But I can’t fault those who do.
Nor will I, but if your left leaning values cause you to cast a vote that's only handing power to folks far, far to the right of Biden/Harris based on Biden/Harris not being left enough for you, don't ask me to pretend that's not exceptionally self defeating.