Senate Forecast 2024

No more teasers

To those of you waiting for the Senate Forecast, bless you. I've had the raw numbers for over a month - I incorporate my Senate data into my Presidential forecast (I'll explain why later for people who care) but I've been a little busy!

Fortunately, I just got the news my book has been sent to print today (no more edits/approvals/questions)

So my side projects will now mostly consist of…promoting the book, and following the Uruguay election - which I'm sure you're exhilarated about.

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No more suspense - here's the headline:

Senate Control Probability

Republicans: 66%

Democrats: 34%

If you had told me 6 months ago that Democrats had anywhere near a 1/3 chance of keeping the Senate, I'd have told you to stop huffing the Hopium.

Keeping ALL of their currently held seats, minus West Virginia, I'd have rated as closer to a minor miracle. Even in a good year, losing a Senate seat in Montana OR Ohio - with Trump on the ballot - is pretty probable.

But that probability has ticked substantially upward from minor miracle to moderate upset.

Sherrod Brown in Ohio is, by my estimate, a medium favorite (64%) and Tester in Montana (33%) a medium underdog.

The simplest outcomes (in no order)

1. Dems keep these seats, plus the “more likely” ones, and end at 50 seats. In this scenario, it's highly likely (greater than 95%) that they have also won the Presidential Election, thus retain control of the Senate.

2. Dems lose Montana OR Ohio, ending at 49 seats. In this case, it's still likely (though far less than 95%) Dems have won the Presidency.

3. Dems lose Montana AND Ohio, ending at 48 seats. In this case, it is more likely that Dems have lost the Presidency, but closer to a tossup.

All of this “if this happens…then this is more likely to happen…then this is more likely to happen…” is why good forecasts account for “covariance.”

Both in the Senate and the presidency.

A scenario where Sherrod Brown holds the Senate seat in Ohio means Democrats have held enough of Ohio's notoriously swingy districts that they've also held in the “blue wall” states for the Presidency.

This statistical reality of covariance is why FiveThirtyEight, flaws and all, produced a much better forecast than anyone else in 2016, because they understood that just because Hillary is “favored” in most of the swing states doesn't mean if she loses one, she's still just as large of a favorite in others.

The only way one could have concluded she was “99%” or anything close to it would be if they didn't account for this. For example:

Deep analysis (that betrays a basic statistical concept).

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Range of outcomes

It's not a pretty chart for Dems, but it could be much worse. They're defending a lot of swing states, and in a Presidential Election those things tend to trend pretty closely.

But it appears that swing state Dem Senators will probably outrun Harris by some number. While running ahead of her by 1 or 3 doesn't feel like much, remember:

• In 2016, EIGHT STATES in the Presidential Election were decided by a margin of 3 or less

• In 2020 SIX STATES in the Presidential Election were decided by a margin of 3 or less

Ballot-splitting is more common in some states than others - a fact that Dems will rely heavily on if they hope to retain the Senate. But a swing state senator running ahead of Harris by just two points could easily yield a Dem Senator in the same state that goes Red for Trump.

And there's a good chance in some of those swing states (even excluding Montana and Ohio as high ballot-splitting states) that the senators run ahead of her by more than that. It isn't a bad thing that Dems have good Senate candidates, but it makes for some weird possibilities.

Republicans are again running some subpar candidates, but they do nonetheless have hope of flipping seats beyond Ohio and Montana: any of Nevada, Michigan, Wisconsin, Pennsylvania are not far-fetched.

This is why the “left tail” for Republicans - an election that goes very well for them - is wider. Democrats are defending a lot of seats. There's about a 1/10 chance Republicans end with at least 53 seats (most commonly, flipping the borderline certainty of West Virginia, plus Ohio, Montana, and one of the swing states).

In the event they've won one swing state (say, Nevada) that means states like Arizona and Wisconsin are closer than my median forecasts.

Which means, their 54th, 55th, and *gasp* 56th seats are now all likely within a couple points.

On the other end, Democrats are surprisingly “in-play” in Texas and Florida, putting 51 and 52 seats into the realm of “not impossible.”

For Dems to get to 51 seats (also about 1/10), the math is much easier: they hold all of their current seats, and win one of the two stretches.

A very good election for Dems (about 5%) sees them picking up both Texas and Florida, and some small number they discover a 53rd seat (most commonly Missouri).

While these “tail” scenarios feel unlikely - and they are - their chances of happening are greater than zero.

Of course, the focus is naturally in the middle.

Unfortunately (for my political leanings) I don't envision a scenario where Dems enter November better than a coinflip to hold the Senate. Such a scenario would require a lot of positive movement in Ohio and Montana, a possibility I consider less than likely (though I would not be shocked if Brown ends north of 80% win probability in Ohio).

I will share more detail soon:

• Vote share forecast for Presidency

• Vote share forecast for Senate

And then later:

• House forecast

• PAR ratings for House and Senate