Our Senate forecasts — both polls-only and poll-plus — tipped toward Republicans late Monday, giving them about a 51 percent chance of maintaining their majority. While that technically makes the GOP a slight favorite, the fight for Senate control remains basically a coin flip. Still, our Senate forecasts have been inching in the GOP’s favor over the past several days, largely driven by a shift in the generic congressional ballot.
The generic ballot — which asks respondents whether they’ll vote for the Republican or the Democrat in their congressional district — informs the FiveThirtyEight Senate models in a fashion similar to how the national polls affect our presidential model. In states where we have limited polling, for instance, the model uses the generic ballot to infer the trend. And the trend right now is good for Republicans. Democrats have a 1 percentage point edge on the generic ballot, their smallest advantage since the beginning of September. Republicans led in the generic ballot in several recent polls, including a some that gave Hillary Clinton a lead in the presidential race.
In other words, the political climate is about neutral according to the generic ballot. In that environment, we’d expect Democrats to win in Democratic leaning states and Republicans to win in Republican leaning states, all else being equal. And that’s exactly what we’re seeing right now in key Senate races: Contests in red states, such as Indiana and especially Missouri, have moved in the GOP’s direction. It’s one thing for Democrat Evan Bayh to win in Indiana when Democrats are ahead on the generic ballot by 5 percentage points, as they were in mid-October. It’s another for him to lead when the generic ballot is basically tied. In Missouri, we’ve seen Democrat Jason Kander hit a wall in the polls; he’s stuck just behind Republican Roy Blunt. Kander may still pull out a win, but that will be much more difficult than it would have before the pro-Republican shift in the generic ballot.
Democrats, meanwhile, are ahead in all the states that are more Democratic than the nation as a whole on the presidential level except for in the extremely tight New Hampshire Senate race. (Things are so close there that the polls-only and polls-plus forecast disagree on it.) If Democrats end up winning New Hampshire, the party will most likely end up with a net gain of four seats, enough for a 50-50 tie in the Senate.
Our Senate models give Democrats a 70 percent chance of controlling the Senate if it ends up split 50-50. That’s because Clinton is about a 70-percent favorite to win the White House, and the vice president breaks a tie in the Senate. If we knew Clinton were going to win, our Senate model would give Democrats a slightly better than 50 percent chance of taking control.