Left wing argument:  missile defense will scare our enemies because they will think we have an advantage over them and then they will be more hostile and will build more weapons and then there will be an arms race which will take away from the money available or the poor and social programs.  We are better off making friends with our enemies even if our enemies..

Right wing argument.  Without missile defenses the enemy will hit us with a missile and we will be defenseless.  The more our enemy perceives that we are weak the more aggressive they will be.

Right and left wing arguments are plausible. 

Left wing incentives to believe left wing argument   
    Want money for social programs.  Don't want to lose it to military spending.

Left wing beliefs that support left wing argument
     Bad guys aren't so bad.  We are bad.