http://fivethirtyeight.com/datalab/senate-update-a-troubling-trend-for-democrats-in-colorado/"Just 10 days ago, Democrats had been benefiting from a string of good polls in Colorado. Since then, the Democratic incumbent in Colorado, Sen. Mark Udall, has seen his situation worsen, with the past five polls showing a lead for Republican Cory Gardner instead.
The chart below tracks polls of Colorado’s Senate race since June 1. It lists both the margin as originally provided in the poll, and the margin the FiveThirtyEight model uses after the various adjustments it applies. A few of the earlier polls were of registered voters, rather than likely voters, so the model adjusts those toward the Republican, Gardner. On the other hand, some of the more recent polls have had a Republican-leaning “house effect,” so the model adjusts those toward Udall."
