According to the San Francisco Chronicle, political marketing consultant Michael Hendrix says Farmville will allow players to post candidate signage and do door-to-door campaigning later this year. Hendrix works for the Michelle Bachmann presidential campaign, and didn't specify whether users could campaign for any other candidates. We've contacted Zynga to find out the specifics.
Also, we should be more specific: We're talking about campaigning for real-life political positions. Not the mayor of Farmville, a position we'd still probably vote to fill with Jimmy McMillan. (Come on, that facial hair? How could we not?)