There is no defined line where a church sermon changes to a political statement, but the IRS can strip a church of its tax-exempt status if the government determines some unclear boundary is breeched.
The Dakota Beacon has my story about a debate on this clouded issue which took place last year. Read it here. http://dakotabeacon.com/entry/robert_m._engstrom_politics_and_the_pulpit_-_church_sermons_targeted_by_irs/
Zingstrom's Blog 