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These mismatches then show up as outlying data points on the regression of *change* in House popular vote versus *change* in national primary margin. But all the other years fall nicely along one tight line.

Regression of change in house popular vote versus change in national primary margin from 1939-2022.

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But because four of the epochs have very similar slopes, we can transform the chart by looking at the *change* in both variables. And when we do that, the epoch divisions transform into mismatches between the two time series.

Graph showing the *change* in margin for both House popular vote and national primary between 1918 and 2022. From 1942 onwards, they closely follow each other, except at epoch changes (1938, 1966, 1982, 2006) and one more potential outlier - 1970.

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Some of these divisions are obvious on the original chart, and some are not.

Graph showing the national primary margin and the House popular vote margin from 1918 to 2022. The are right on top of each other from 2006 to 2022, but before that they have a significant but varying gap.

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... this plot hides within it four epochs with nearly the same slope (and one epoch that is a mess, 1918-1938). It's kind of like a Simpson's paradox (not sure if it counts since it doesn't reverse the sign of the slope).

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There's still more to show in the relationship between primary votes and the House popular vote! You can plot the data in graph below as a scatterplot. But a) it's obvious from the original graph this is a bad idea and b) it's not too helpful if you want to predict things. But...

A scatterplot of House popular vote margin versus national primary margin in midterm elections from 1918 to 2022. While there is a decent regression for the data, they fall in a wide band, such that any prediction you try to make will not be helpful. For instance, this year we would expect a House popular vote margin between about R +15 or so and D +12 based on the primary margin of D +11. Not useful at all!

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This is why diesel prices - which selectively affect Trump voters and rural areas in particular - matter for the election. www.kff.org/public-opini...

Chart showing that 59% of Republican rural voters who are dissatisfied with Trump on the economy are absolutely certain to vote, compared 79% of Republican rural voters who approve of Trump on the economy.

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From the NYT/Press Herald/Siena Maine poll crosstabs: Maine likely voters prefer Democratic control of the Senate by 55-43. Including... 13% of those who are voting for Susan Collins. www.nytimes.com/interactive/...

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If we instead look at the data available in July of election year, both for primaries and generic ballot polls, the predictions based on primaries are much better than the generic ballot polls.

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We can see both measures do a very good job since 2010; in the 20th century, the generic ballot polls were a little worse, especially mid-century. However, this uses the *final* generic ballot polls, which aren't available until election eve. What about earlier?

Graph showing house popular vote, November generic ballot polls, and the predicted house margin from primary votes.

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If we could go back in time to use the national primary margin to predict the midterm House popular vote with the data available at that time, how well would it have done? Pretty well overall, with some significant misses like 2006. (Note we see no warning signs of a miss like 2006 for this year.)

Graph showing the actual House Popular vote margin compared to that predicted by the primary margin. Major trends match, and about half the time the prediction is very close.

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Finally, in the Northeast, Democrats have long outvoted Republicans in primaries, but never so much as they have done this year. The 1930s surge is still prominent but subsequently subsided as in the Midwest.

Primary votes from 1918 to 2022 in ME, NH, VT, MA, RI, CT, NY, NJ, PA, MD, and DE.

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The Midwest has flipped back and forth for a century, with no party dominant for decades at a time. The Progressive Era end looms large, with Democrats quadrupling their primary vote total from 1934 to 1938, but then falling back again. Recent increases in vote totals are more muted but still clear.

Primary votes from 1918 to 2022 for IA, MN, WI, IL, IN, and OH.

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The rest of the West also saw the Democratic primary vote rise from nearly nothing in 1934, but then it continued to keep pace with population. Dem numbers have skyrocketed since 2014, outpacing increases in Republicans and this year yielding the highest collective Democratic margin since 1974.

Primary votes from 1918 to 2022 from HI, AK, WA, OR, CA, NV, AZ, NM, and CO.

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Outside of the South, only one other subregion shows Democratic fortunes fading to any extent in recent years, and that is in the Northern Plains and Northern Rocky Mountains. In these states, 1934 jump-started the fortunes of the Democratic party, followed by a plateau.

Primary votes from 1918 to 2022 in UT, ID, MT, ND, and SD. Republican votes have surged in recent years, while Democratic votes have remained at roughly the same levels since the 1940s.

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The Upland South, on the other hand, does not show a recent Democratic renaissance. The decrease in primary margin from 2018 may be an artifact due to chance, as there are only two states in this group which elect their governors in midterm years, yielding a less consistent dataset.

Primary votes in the Upland South: WV, KY, TN, AR, MO, and OK from 1918 to 2022

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Yesterday we showed how important primary turnout can be on a national level. But there's interesting things going on at the regional level, too. Most notably, in the core of the South, Democrats *outvoted Republicans* for the first time in decades in the primaries! (thread)

Total primary midterm votes in the core of the South from 1918 to 2022 showing a recent Democratic surge. States include VA, NC, SC, GA, AL, MS, LA, and TX.

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Interestingly, the two states with the next-best track records in this regard are... NV and SC. Is this a coincidence? I don't know! Maybe being an early presidential primary state makes voters more attuned to national politics.

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And the regression here is also super tight... but it predicts a 2026 House popular vote of D +1. Either both of these predictions will be pretty wrong, or one will be very, very wrong. This illustrates how correlations in politics can fall apart, even if you have beautiful r-squared values.

Regression with 9 data points and R2 of 0.96

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But Iowa has an even longer track record of the primary margins nearly matching the behavior of the House popular vote!

A graph showing the changes in primary margin in midterm years match the changes in House popular vote margin for Iowa. While the magnitude differs, the direction and relative change is nearly identical.

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This gives us a *very* tight regression which predicts a midterm result this year of D +9.

Regression with 6 data points and R2 of 0.97. The 2026 value is beyond the edge of the regression to the right.

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Yesterday we showed you how the sum of primary votes in all the states can predict midterm election outcomes surprisingly well. But that's not all! There's also some individual state primaries that appear to be bellwethers. Here is New Hampshire since 2002. Note the two different axes.

A graph showing the changes in primary margin in midterm years match the changes in House popular vote margin for New Hampshire. While the magnitude differs, the direction and relative change is nearly identical.

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Outside of the South, however, the two measures, on average, follow each other a little better, with the exception of the most recent midterms.

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In 1970 we see the House popular vote margin rise while this time the national primary vote margin falls. Unlike 1966, the phenomenon seems entirely confined to the South.

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Outside of the south, the outliers disappear! The relationship stays about the same though.

A graph showing the change in the margin of the national primary for all states outside the south and the change in the margin of the house popular vote, from 1918 to 2022. The two lines follow each other fairly closely from 1946 to 2022.

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Alan Abramowitz has modeled this! It would be wild... centerforpolitics.org/crystalball/...

A table showing predicted Senate seat swing as a function of Dem margin on the generic ballot, varying from D +3.6 seats at a generic ballot margin of D +2 all the way to D +7.3 seats at a margin of D +12

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And sometimes, diesel is used in personal vehicles chosen purely for... aesthetic reasons. The owners of these vehicles, well, they made their choice.

Reddit comment saying "Do I care when people tell me I don't need a diesel anymore and I should sell it because I don't have any trailers anymore? Absolutely fuckin not I would never ever buy a gas powered American truck, so no I'm not selling it. It's mine, I earned it, I paid for it. Buy what you want, drive what you want.

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Diesel is used for construction equipment, trucks for hauling and towing, and delivery vans; the small business owners of the Trumpian petite bourgeoisie pay when the price goes up. www.cambridge.org/core/journal...

Chart showing diesel-intense small businesses like construction and trucking are 60-80% Republican owners

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Diesel is used for farm equipment; rural landowners pay when the price goes up. www.nass.usda.gov/Charts_and_M...

Chart showing farm expenditures on diesel at about $6000-$9000 per farm per year for the years 2015 to 2024

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