Nate Silver --- FiveThirtyEight
The battle for Senate control has been close all year, but also remarkably consistent. Way back in March, we described Republicans as slight favorites to pick up the chamber. And since FiveThirtyEight officially launched its forecast model this month, Republicans have had between a 53 and a 65 percent chance of winning the Senate. Our most recent update, as of Thursday evening, is close to the middle of that range, putting Republicans’ takeover chances at about 58 percent.
There’s no guarantee things will remain this way. At just about this time two years ago, Democrats broke open what had been a stalemate in the battle for Senate control. The FiveThirtyEight forecast had the race almost even at the beginning of September 2012, but had Democrats as an 80 percent favorite by the end of that month.
This year, however, has been characterized by what Charlie Cook calls “head fakes.” Just 10 days ago, Democrats had been benefitingfrom 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.