Sam Wang compares this election to past ones:
Journalists and pundits have lavished considerable attention on the question of who will control the Senate in 2015. But a broader phenomenon has escaped notice: the sheer number of close state-level races, both in the Senate and in statehouses. At risk are many incumbents who were elected in previous wave years: in 2010 for Republican governors and in 2008 for Democratic senators.
Silver remarks upon the relative stability of his forecast:
Republicans’ odds have never been higher than 66 percent — a figure they reached late last week — or lower than 53 percent. The informal model updates we published going as far back as March also had Republicans as 55 or 60 percent favorites.
To an extent, this stability reflects the noise-reducing features of the FiveThirtyEight model. Our program examines the polls for signs of statistical bias, and weighs them more heavily when they have larger sample sizes, better methodologies and better track records — which can reduce the impact of outliers. The FiveThirtyEight model is also fairly conservative in estimating the uncertainty associated with each race and the disposition of the Senate overall. At times in the past, the polls in most swing states have been biased in the same direction (either toward Democrats or Republicans).
But this degree of stability is unusual. In pretty much every election we’ve covered, the polls have more clearly broken toward one or another party by this point.
Nate Cohn anticipates that “the results of the governors’ races may ultimately play a big role in how analysts interpret the results Nov. 4, including whether this will be viewed as a ‘wave election.'”:
On balance, Democrats seem set to pick up two or three states, mainly because the Republicans enter the elections with twice as many Republican-held seats. But it is easy to imagine the Republicans holding their advantage — there are 29 Republican governors and 21 Democratic ones — or the Democrats picking up a half-dozen seats.