Senate Now a ‘Toss Up’ According to Nate Silver’s FiveThirtyEight Model

My, that evolved rapidly. Just days ago the FiveThirtyEight forecast was pretty firmly saying that, at best, Republicans would hold the line and keep the Senate at a 50/50 split.

Now, with just about two weeks to go, Nate Silver has revised his Senate forecast to call the odds of GOP control basically a “toss up” as more races move away from likely Democratic holds. Just about two weeks ago we examined the difference between RealClearPolitics, which is forecasting a pretty decisive GOP Senate victory, to the FiveThirtyEight Senate model, which was more kind to Democrats.

Back in September, Silver was on Twitter warning that his House or Senate model could be wrong based on … more modeling that shows it could be wrong. Whatever the case, the tone has changed as more and more data forecasts a potentially big election night for the GOP:

But the main reason why I think of the race for control of the Senate as a toss-up — rather than slightly favoring Democrats — is because there’s been steady movement toward the GOP in our model over the past few weeks. In principle, past movement shouldn’t predict future movement in our forecast and it should instead resemble a random walk. (We put a lot of effort in our modeling into trying to minimize autocorrelation.) This year, though, the forecast has moved in a predictable-seeming way, with a long, slow and steady climb toward Democrats over the summer, and now a consistent shift back toward Republicans.

The summer produced an unusual streak of favorable developments for Democrats, from the backlash to the Supreme Court’s decision to overturn the constitutional right to abortion, to lower gas prices, to former President Donald Trump’s unhelpful involvement in the midterms, to a string of legislative victories for President Biden. There is, of course, a tendency to see patterns in random noise, but it’s as though the coin really did come up heads for Democrats six or seven times in a row.

I’ll take a stab and sum up why the FiveThirtyEight Senate model came up “heads” for Democrats throughout August, September, and much of October. It’s the old adage of “garbage in, garbage out.” The model is based on polls, and polls of state-level races can vary wildly in terms of accuracy from state to state and especially from week to week. If the data is wrong underneath then no amount of adjustments will account for bad numbers to start with.

Silver puts a lot of effort into modeling and a lot of effort to adjust polling weight, countering bias, correcting for possible polling errors, and outright “gut” instinct over what the data is trying to say. If you start with weak data, however, or perhaps incomplete data, then the best you’re getting is an incomplete picture.

As Election Day creeps closer, the numbers have lurched toward the GOP and the Senate forecast seemed to shift almost overnight in the FiveThirtyEight model:

According to Silver’s long explanation, he now believes the model is still over-selling chances of Democratic control in November. Quite an about-face from just a few weeks ago.

For what it’s worth, the RealClearPolitics forecast has gotten rosier for Republicans as well, now showing a potential 53/47 split as RCP predicts the state of Georgia might end up flipping as well.

All taken with a grain of salt since the only poll that matters is the one taken on Election Day, after all.