Good New Yorker story on Gallup history (they have manipulated polls before)
The polling organization Gallup created a stir earlier this month when it announced that, going forward, it would refine its polling methodology, emphasizing the preferences of voters considered most likely to vote. The immediate consequence of the shiftthe obliteration of a five-point advantage for Obamahas raised the hackles of pundits and the public at large. Some, like Slates Sasha Issenberg, have criticized the shift, claiming that likely voters can sometimes lie. Others have questioned why Gallup waited until four weeks before the election to make the change. Theres been a lot of argument about the usefulness of polls in general. Perhaps Gallups polls are woefully inaccurate; perhaps thats true of most polls...
In fact, in 1944, Gallup had tried to resolve this difficulty in a different way: by simply giving Republicans a two-percentage-point advantage in its polls. The organization failed to disclose this step publicly, and there was outcry when it was later revealed at a Congressional hearing.
Read more
http://www.newyorker.com/online/blogs/backissues/2012/10/george-gallup-and-the-mystery-of-polls.html#ixzz2AoYuPrT5