I think this is could be a rational matter of trust. You can't be an expert on understanding every proposal. You have one political representative group that has the appearance of being relatively aligned with your values, and another group that seems to be actively against many of your values. Which representative group are you going to extend some benefit of the doubt or delegate to, and which are you going to be more skeptical of?
But it's not always a matter of trust, it can also be a matter of attack. Consider one political party blocking all initiatives by the other party, to deny the other any wins, even if the initiative is also in the interests of their own party. Or consider the "own/trigger the libs" behavior, where the goal seems to solely be to hurt, not to advance any other goal. On the other end of an attack, though, they'll tend to have less trust of anything from any group they associate with the attack.