Ask them if there’s anything Biden could do to make them not vote for him. Just a yes/ no, don’t ask them for their specific red line.
If the say no- ask them how that makes them any different from Trump voters (they criticize his voters for following him blindly and the worst thing that can happen to them is being compared to a Trump voter.)
If they say yes; then it’s just degrees of difference from you and them.
My first job out of college was a shitty sales job… the first two weeks were literally me and 6 other new hires writing down a script based on the boss reciting the approved responses and it was our job go home and type them out and then print out and have them at our desk for then spending 8 hours on the phone. Insane in hindsight