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.

There’s no indication that herding is necessarily done on purpose to fake numbers—but rather it’s an effort to correct mistakes in data samples.