store the selected state in a session variable.
If you are using the checkout page that is created by the checkout wizard, the state will already be stored in session variable named "eCartCheckoutForm_state_province"
you can create a discount rule that uses this session as a trigger:
Based On Session Variable value:
if the Session Variable "eCartCheckoutForm_state_province" has a value "=" to "OR"
OR
if the Session Variable "eCartCheckoutForm_state_province" has a value "=" to "WA"
OR
if the Session Variable "eCartCheckoutForm_state_province" has a value "=" to "ID"
then set the discount calculation to the desired discount.