The 2012 election was within the margin of error and Rassmussen 'blamed' their inaccuracy on a higher than expected (more than 2008) non-white turnout. In 2008 the non-white voter was 26% of the vote, in 2012 it was 28%....
http://www.politico.com/blogs/charlie-mahtesian/2012/11/rasmussen-explains-149240