The recent data from the PEW foundation on political attitudes shows party positions becoming more divergent. The trend reminds me of social psych research on communication and attitudes. If you determine existing attitudes and then group individuals for discussion either with mixed or similar attitudes, you find the average position in groups with similar attitudes become more extreme. So, the PEW outcome would be produced by a pattern of information access selected to be consistent with existing positions. Consider your selection of a cable channel for news.