@rosborne979,
That's not what I believe. I believe that churches, like all other tax-exempt non-profits, cannot influence elections by specifically endorsing candidates or specifically attacking their opponents. They can, however, take sides on issues, and comment on individual politicians' stand on these issues.
But it doesn't matter what what I believe, so I searched the web for "churches, political, tax-exemptions". (Frankly I think that would have been your job, since you're the one who made confident political statements in the matter.) The first hit was
this page on atheism.about.com, which basically confirms my view. (About.com provides context on several other pages. See
this one for cases about when it's okay for the IRS to revoke a church's tax exemptions;
or see this one for churches who lost their tax exempt status for inappropriate intervention into political campaigns.)
Assuming that About.com describes the law correctly -- and I suppose they do, as their articles on this topic appear consistent and well-researched to me -- the verdict would seem to depend on how the courts interpret the church's behavior. If they interpret it as a campaign to get pro-life candidates elected, the IRS could revoke their tax-exempt status. If they interpret it as a campaign about the issue of abortion, that would be a legitimate form for the church to influence politics.
That leaves the threat of excommunicating pro-life politicians, and of the Catholic Church threatening that it might do so. I'm not a lawyer, but I see little hope for the politicians here. If
the Boy Scouts of America have a civil right to expel homosexuals from their ranks, I don't see how the Catholic Church does not have a right to expel members who work against one of its central doctrines.