The following post is by Alan I. Abramowitz, author of The Polarized Public:
From the Democratic Strategist—For the past 8 months, Rasmussen has consistently found more Republicans than Democrats in its national party id polling and that's among adults, not likely voters or registered voters. They're virtually alone in this regard and well off of the overall polling average which has a D lead of about 7 points with Rasmussen removed.
Do we need any more proof of Rasmussen's Republican slant? This isn't just a "house effect." Rasmussen is a shill for the Republican Party as is clear from the wording of many of the questions he asks as well as his results. For more evidence, see Nate Silver's 2010 polling post-mortem in which Rasmussen received the worst rating of any major polling organization for accuracy and bias:
As in 2010, Rasmussen is 'flooding the zone' by releasing numerous state and national polls, thereby strongly influencing polling averages because of the frequency of his polls and their outlier results.