There is some good news for McWherter -- FiveThirtyEight sees the election growing tighter by Nov. 2, predicting a final margin of victory for Haslam of 55.0 percent to 42.2 percent.
According to the site, much of the forecasting model relies on its successful Senate forecasting model: "The basic blueprint of taking a robust average of polls, supplementing them with some demographic information and then using them to project the final standing of the candidates remains the same." There are tweaks, including the assumption that races between nonincumbents for governor tighten as election day draws closer.
Things can change. In the 2008 presidential election, which FiveThirtyEight nailed (it only missed Indiana), the final few months of the election showed definite swings, with John McCain at one point, during and after the Republican convention, looking more likely to overtake Barack Obama. Alas for McCain supporters, that evaporated after the financial crisis and the first presidential debate at Ole Miss.