Mark Blumenthal: "Let's be clear: It is always sensible to treat sponsored, internal surveys with extra skepticism when they are publicly released. Political scientists that have studied public polls (examples here and here) find that partisan surveys typically show a an average bias of 2 to 4 percentage points favoring the sponsoring party. One reason for this phenomenon is that most internal polls never see the light of day. Campaigns typically choose to share only those polls showing good news, not bad."