PPP (Public Policy Polling) is a Raleigh, North Carolina polling company and according to Nate Silver, PPP is the worst by putting their thumb on the scale.

http://fivethirtyeight.com/features/heres-proof-some-pollsters-are-putting-a-thumb-on-the-scale/

Silver goes on to say that herding increases the closer you get to election day. 

And, concludes that PPP herds toward the polling average more than other pollsters.