A grand idea, but as the saying goes, who bells the cat?
Churches are seen as a normal part of life, and while church exemption from taxes may not quite be up there with hot dogs, apple pie and Ford (I'm not a Chevy person!), it is for the large portion accepted as the way things are by American culture at large. To date there have been some pushes made by atheists to rescind the tax-free status of those churches which blatantly endorse certain candidates for office, but those actions (to my knowledge) have gone nowhere so far. It's also worth keeping mind that the average politician is as married to the idea of church-tax exemption as their constituency is. Any push aimed at them will likely be met with a push BACK.
Don't misunderstand me, I'm in favor of taxing the churches. I just think we're going to need far larger numbers on our side before the idea is liable to have any traction.