It is not just churches that will be harmed by a change in the deductibility of charitable contributions. If anything the churches will see the least harm of any group by such a change.
Those who will see the most harm are non religious organizations such as local museums, cultural centers including art museums, theatrical production groups, botanical gardens, community centers, etc. lets also not forget the local square dance or polka club and any hobbyist groups such as model airplane enthusiasts that have a flying field open to the public or the live steam model railroad group in the community park in many of our larger cities. These groups will all see reductions in their ability to support public programs as their funds go to some po