Meanwhile, the IRS sent out a letter late last year warning churches about not straying over the line, and providing lots of useful info on how to play by the rules.

The Internal Revenue Service today reminded section 501(c)(3) organizations, including charities and churches that federal law prohibits them from becoming directly or indirectly involved in campaigns of political candidates.

The prohibition against political campaign activity has been in effect for more than half a century and bars certain tax-exempt organizations from engaging on behalf of or in opposition to political candidates. However, these organizations can engage in advocating for or against issues and, to a limited extent, ballot initiatives or other legislative activities.

This is a perennial issue that is perennially poorly understood by most, even ordinarily well-informed people.  Here are some key points from the
IRS guidelines on this:  

The Prohibition on Political Campaign Intervention

Under the Internal Revenue Code, all section 501(c)(3) organizations are absolutely prohibited from directly or indirectly participating in, or intervening in, any political campaign on behalf of (or in opposition to) any candidate for elective public office.  The prohibition applies to all campaigns including campaigns at the federal, state and local level.  Violation of this prohibition may result in denial or revocation of tax-exempt status and the imposition of certain excise taxes.  Those section 501(c)(3) organizations that are private foundations are subject to additional restrictions that are not described in this fact sheet.

What is Political Campaign Intervention?

Political campaign intervention includes any and all activities that favor or oppose one or more candidates for public office.  The prohibition extends beyond candidate endorsements.  Contributions to political campaign funds or public statements of position (verbal or written) made by or on behalf of an organization in favor of or in opposition to any candidate for public office clearly violate the prohibition on political campaign intervention.

Distributing statements prepared by others that favor or oppose any candidate for public office will also violate the prohibition.   Allowing a candidate to use an organization's assets or facilities will also violate the prohibition if other candidates are not given an equivalent opportunity.   Although section 501(c)(3) organizations may engage in some activities to promote voter registration, encourage voter participation, and provide voter education, they will violate the prohibition on political campaign intervention if they engage in an activity that favors or opposes any candidate for public office.  Certain activities will require an evaluation of all the facts and circumstances to determine whether they result in political campaign intervention.

Voter Education, Voter Registration and Get Out the Vote Drives

Section 501(c)(3) organizations are permitted to conduct certain voter education activities (including the presentation of public forums and the publication of voter education guides)  if they are carried out in a non-partisan manner.  In addition, section 501(c)(3) organizations may encourage people to participate in the electoral process through voter registration and get-out-the-vote drives,  conducted in a non-partisan manner. On the other hand, voter education or registration activities conducted in a biased manner that favors (or opposes) one or more candidates is prohibited.

How heavy a hand the IRS should bring to these matters is also a perennial question. But the religious right has gotten a pretty free ride for a long time, but he IRS seems to be making an extra effort to ensure that the law is fairly and uniformly enforced.

The key for most people is to understand that the IRS does not launch independent investigations. The agency responds to specific complaints filed by citizens and watchdog groups. Americans United for Separation of Church and State has long been the leader on these issues. More recently, CREW and Catholics for a Free Choice, among others, have been active as well.