Because our political choices are ethical choices, we cannot separate our politics from our religious beliefs. Thus, Catholics and other Christians have always drawn lines based upon Biblical principles before the dreams of ambitious politicians.
Note that these are not lines we dare our leaders to cross. Instead, these are lines we dare not cross. When our leaders demand that we participate in that which we know to be sinful, we have only one option. We must dig in our heels. We must refuse, and we must oppose them.