Fr. Martin Fox, who prior to entering the priesthood worked in political lobbying/advocacy, has an interesting post (and comment threat) up about money in the political process. His basic thesis is: It's not so much the case the politicians change their beliefs in order to get money from donors (though that happens at times) but rather that candidates with certain types of beliefs are more likely to get money from one source or another or no source at all, depending on who is interested in supporting those beliefs.
Thus, one candidate may raise a great deal of money from big labor as a result of voicing his beliefs in favor of protectionism and high wages, while another candidate raises lots of money from large businesses as a result of supporting open boarders and 'right to work'. However, it is the beliefs that come first and the money that follows after.