According to whom? None other than the Pew Research Center, a left-of-center organization. Moreover, Pew's latest survey only reaffirms previous surveys demonstrating the same result.
In fact, the results weren't even close.
In a scientific survey of 1,168 adults conducted during September and October of last year, respondents were asked not only multiple-choice questions, but also queries using maps, photographs and symbols. Among other subjects, participants identified international leaders, cabinet members, Supreme Court justices, nations on a world map, the current unemployment and poverty rates and war casualty totals.
In a 2010 Pew survey, Republicans outperformed Democrats on 10 of 12 questions, with one tie and Democrats outperforming Republicans on just 1 of the 12. In the latest survey, however, Republicans outperformed Democrats on every single one of 19 questions.
Amusingly, the Pew report attempted to soften the stark partisan knowledge disparity:
"Republicans generally outperformed Democrats on the current quiz. On 13 of the 19 questions, Republicans score significantly higher than Democrats and there are no questions on which Democrats did better than Republicans. In past knowledge quizzes, partisan differences have been more muted, though Republicans often have scored somewhat higher than Democrats."