I agree, largely, that DQ's argument taken to its logical conclusion would turn out badly. The same is true though if we allowed churches to become too involved in candidate endorsement, though. That is, they would be prefenced over PAC's (due to the tax treatment of donations to them) and probably turn into full blown political parties if there were no limits on their ability to engage in politics. Anybody that hasw any respect for the good churches do (and I do) should be bothered by that thought.
I would prefer the churches follow the law as it exists and not try so hard to force a conflict. If they insist then I would argue for removing their tax exmption as it would be preferrable to seeing them being perverted into a political organization.
The reason unions are given some leeway is because that IS their interest. In other words, if a union could not support various laws and policies it's abilities would be greatly circumscribed. I don't see why they should be able to explicitly endorse candidates, though. But, maybe, they get around with a PAC or some other form of organization that acts outside the bounds of their usual union services. I don't know.