Rasmussen Reports has become a controversial polling outfit in this election cycle, for two reasons: a massive increase in the amount of polling it produces and its newfound "house effect" in favor of Republican candidates.
First, during 2009-2010, Rasmussen Reports has produced has drastically increased its quantity of polling. For example, in addition to their daily tracking poll on President Obama's job performance and their weekly poll on the generic congressional ballot, 28% of all the polls conducted on the 2010 Senate elections, up from 18% in 2007-2008.
Second, and most importantly, Rasmussen has become controversial because its massive amount of polling has skewed decidedly in favor of Republican candidates. Rasmussen surveys have favored Republicans by 5.5% compared to other polling firms so far in 2009-2010. In 2007-2008, Rasmussen was in line with all other surveys, showing no House effect at all.
As Nate Silver showed over the weekend, this newfound skew in favor of Republicans is not simply the result of Rasmussen surveying likely voters while other polling firms are looking at registered voters or all adults. Comparing apples to apples, Rasmussen's likely voter generic congressional ballot polls favor Republicans by 6% compared other likely voter generic congressional ballots. Further, Rasmussen's numbers for partisan identification among all adults also favor Republicans by about 6% compared to other polls that measure partisan identification among all adults.
To reiterate: Rasmussen's likely voter polls are skewing 6% in favor of Republicans compared to other likely voter polls, and Rasmussen's polls of all adults are skewing 6% in favor of Republicans compared to other polls of all adults. So, this has nothing to do with an enthusiasm gap or a likely voter screen. Rasmussen is simply flooding the zone with polls that skew towards Republicans by an average of 6%.
The most likely response defenders of Rasmussen polls will make to these numbers is to point at Rasmussen's strong track record of predicting election results from 2004-2008. However, I am reminded of an earlier election when Rasmussen erred massively due to an enormous, pro-Republican "House effect." Back on November 7th, 2000, when operating the "Portrait of America" poll, Rasmussen projected Bush to win the national popular vote by 9% over Gore, 49%-40%. The average of the other 11 polls in the national trial heat only put Bush up by 1.5%, which means that Rasmussen has a "house effect" of 7.5% in favor of Republicans:
The difference between Rasmussen and all other polling firms in 2000 sure looks a lot like the difference between Rasmussen and other polling firms now.
Finally, even though I don't enjoy making these implications, it is worth noting that in the 2000 election, Rasmussen's primary media sponsor was World Net Daily. While I have absolutely no proof at all that WND's sponsorship of Rasmussen played a role in Rasmussen's massive, outlying house effect in favor of Republicans, it certainly does raise an eyebrow or two. Similarly, in 2009, Rasmussen scored significant new capital investments. Again, even though there is no apparent connection between the new, pro-Republican "house effect" in Rasmussen and their new financial partnership, it raises eyebrows.
Calling veracity of individual polls into doubt has an increasingly long, but very checkered, history. From 2004-2008, simply averaging the results from almost all polls over the final 15 days of a campaign, without any weighting at all, has proven to be the most reliable method of forecasting general election results. Zogby Interactive, Strategic Vision, and Columbus Dispatch polling are the only current examples of polls that are so inherently flawed it is better to just ignore them altogether. I seriously doubt that Rasmussen will end up on that scrap heap, but given their vastly increased quantity of polling, their newfound (or re-emerged) pro-Republican "house effect" can't simply be dismissed. Something has happened to Rasmussen polling, and more answers are needed.