Rev. Rul. 200741
Organizations that are exempt from income tax under section 501(a) of the Internal Revenue Code as organizations described in section 501(c)(3) may not participate in, or intervene in (including the publishing or distributing of statements), any political campaign on behalf of (or in opposition to) any candidate for public office.
26 CFR 1.501(c)(3)1: Organizations organized and operated for religious, charitable, scientific, testing for public safety, literary or educational purposes, or for the prevention of cruelty to children or animals.
http://www.irs.gov/pub/irs-tege/rr2007-41.pdf
Would make a nice bumper-sticker.