I was struck by the wild swing in numbers in the recent Real Clear Politics Presidential tracking poll. The most recent Rasmussen poll has Romney up by four points, while the last poll conducted by Fox (just one week ago) has Obama up by nine points. A 13-point swing. Purple Strategies has Romney leading in Ohio, while most other polls still have Obama ahead in Ohio and key battleground states, which Purple Strategies claims it targets.
I think you have to look at the persons behind the polls to get a better understanding of the wild fluctuations. Purple Strategies was recently co-created by Alex Castellanos, a leading Republican strategist who worked for Bush's '04 and Romney's '08 campaigns. Hardly a bipartisan survey taken here, but one designed, it seems, to skew the results of the popular RCP index, which takes an average of leading polls.
Rasmussen Reports is also relatively new (2003) and was created by Scott Rasmussen, who along with his father, William, created ESPN. Both are long-time Republican supporters, so not surprisingly Rasmussen generally favors Republican candidates.
No poll is without bias, especially given the relative limited number of persons it surveys. The Gallup Poll has a long history, and its founder George Gallup was determined to keep the poll above partisan political influence, but in 1988 the Gallup family sold the firm to Selection Research Incorporated (SRI), based out of Omaha, Nebraska, which appears to keep a low profile.