If they get involved in specific ways in politics, they can lose their tax exempt status.
Of course, only preachers with a liberal message will be pursued under this rule with this administration regardless of how many churches were used to recruit GOP voters.
http://www.washingtonpost.com/wp-dyn/content/article/2005/11/18/AR2005111802501.html
"The Internal Revenue Service is examining the tax-exempt status of a liberal church in Southern California because its former pastor delivered a fiery antiwar sermon that criticized President Bush by name on the Sunday before the 2004 presidential election."
"Under federal law, religious groups and other nonprofit charitable organizations that qualify for tax exemptions under Section 501(c)3 of the tax code may not "intervene in . . . any political campaign on behalf of (or in opposition to) any candidate for public office."
http://www.lc.org/resources/pastors_churches_politics.htm
Do a yahoo search for:
church tax exempt status politics bush
you will get tons of hits