He's risking a superb reputation as a political prognosticator by predicting a 70 percent chance of Democrats holding the Senate, while the NYT's Upshot, WaPo's Monkey Cage and Nate Silver's FiveThirtyEight, well, are not. Wang's predictive emphasis is on polling. He explains:
I categoriz[e] models as "Fundamentals-based (Type 1)" and "Polls-based (Type 2)". The major media organizations (NYT, WaPo, 538) have all gone with a hybrid Type 1/Type 2 approach, i.e. they all use prior conditions like incumbency, candidate experience, funding, and the generic Congressional ballot to influence their win probabilities--and opinion polls.
However the Upshot, for one, recently increased its odds to a 67 percent chance of a Republican takeover, explaining that
our forecast is ... at its most Republican-leaning point yet this cycle. So perhaps it is worth exploring why.
Recent polls are one big reason.