There's a good article entitled "Tempted by Politics" by Mark Galli here.
Additional reasons for pastors to avoid stumping for political candidates is:
1. Publicly siding with particular parties or candidates can erect a real obstacle for those supporting other parties or candidates from wanting to hear anything else you have to say including the message of the gospel.
2. Participation in politics takes away from the priority of prayer and the ministry of the word (Acts 6:3)
2 Tim 2:4 "No one serving as a soldier gets involved in civilian affairs—he wants to please his commanding officer."