Richard J. Wood (Capital) has published Pious Politics: Political Speech Funded Through I.R.C. § 501(C)(3) Organizations Examined Under Tax Fairness Principles, 39 Ariz. St. L.J. 209 (2007).  Here is part of the Conclusion:

Professor Michael Hatfield starts his article, Ignore the Rumors-Campaigning From the Pulpit is Okay: Thinking Past the Symbolism of Section 501(c)(3) [20 Notre Dame J.L. Ethics & Pub. Pol’y 125 (2006)] by saying that, “[t]here is no law that prohibits churches from campaigning. There is no law that prohibits pulpit endorsements or full page newspaper ads for the political candidate of a church’s choice. Doing such would cost a Tax Exempt church its exemption under 501(c)(3). But that is all.” I hope that I have demonstrated that tax fairness policy supports the conclusion that 501(c)(3) religious organizations should not be permitted to endorse political candidates.