It's the preachers who control it. Basically, the party official goes to the church and makes a contribution to the collection (and he's already given a payment to the preacher) and then preacher then gets up and makes some speech, saying its the Christian duty to vote for whomever.
I have always had a problem with the black church. I feel that the amount of misguided control, by un- or undereducated 'preachers' who are out to line their own pockets, has hampered the growth of the black community. Particulary, those who encouraged their flock to suffer the indignities of racism in order to build up 'treasures in heaven,' can in no way be considered leaders.
I grew up with people who belonged to these types of churches and who were mind-controlled through them. I never understood why some people are afraid to think for themselves. If you are against abortion, pro-family, pro-working hard, and pro-clean living, how in the name of all that's holy do you vote Democrat, no matter what some so-called preacher tells you to do?