The New York Times recently had an analysis of changes in county-level voting between 2012 and 2024. As they summarized it, Trump's victory in 2024 was "the culmination of continuous gains by Republicans in much of the country each time he has run for president, a sea of red that amounts to a flashing warning sign for [the] Democratic Party." Although there were a lot of interesting things in the story, I think there was a basic problem with their analysis. They looked at what they called "triple-trending" counties: that is, counties that had moved in the same direction politically in 2016, 2020, and 2024. There were about 1,500 counties that moved towards Trump in all three elections, and only 57 that moved towards the Democrats in all three. Of course, most of the Trump-trending counties had small populations, but some were large, and they had a combined population of over 40 million, compared to 8 million for the Democratic-trending counties.
The problem is the focus on the "trend." In 2012, Obama got 51% of the vote and Romney got 47.2%, for a Democratic lead of 3.8%. The Democratic lead was 2.1% in 2016, 4.5% in 2020, and -1.5% (48.2% to 49.7%) in 2024. So in the nation as a whole, the changes from one election to the next were -1.7,+2.4, and -6.0, and the change from 2012 to 2024 was -5.3. Suppose you had a county that was tied in 2012, then was -1.5 in 2016, -3.0 in 2020, and -4.5 in 2024 and another county which at zero in 2012, -20 in 2016, -17.5 in 2020, and -23.5 in 2024. The first one would qualify as triple-trending and the second would not, but the second is clearly the one that was more affected by Trump.
Those are just hypothetical examples--what about real counties? The article didn't include a list of triple-trending counties, but I identified a few. One (Pike County, Ohio) was on a list of counties that have moved most strongly Republican over 2012-24. I haphazardly chose five (Hudson, NJ; Cuyahoga, OH; Palm Beach, FL; Clark, NV; and Grant, WI) to represent different parts of the country. A few days later, another Times story mentioned the small town of Kennett, MO and said it was a strongly pro-Trump area, so looked up its county (Dunklin) and found it qualified. The figures show the Democratic margins in presidential elections since 1972:
Dunklin, Grant, and Pike Counties all moved strongly towards the Republicans in 2016. They continued moving Republican in 2020 and 2024, but if you were to pick the election that stood out it would be 2016--the first one that Trump was on the ballot. Trump clearly did change voting patterns in these counties.
Hudson, Clark, Cuyahoga, and Palm Beach are different. None of them had particularly large changes in 2016. Palm Beach, Clark, and especially Hudson had large shifts towards the Republicans in 2024, but if you want to explain that, the natural place to look is at something unique to 2024.
Another way to look at it is to plot the margin in a county against the margin in the national vote. For Pike:
The three Trump elections clearly stand out. For Hudson:
The Trump elections don't stand out. I won't repeat the figures, just say that the Trump elections also stand out for Dunklin and (less strongly) Grant, but not for the others.
I'm not just making the obvious point that "triple-trending" Republican counties aren't all the same--I'm saying that they don't represent a Trump effect, even in a rough sense. If a county moved strongly towards the Republicans the first time Trump was on the ballot and stayed there the next two times he was on the ballot, then it's reasonable to think that he had an effect, even if it followed the national trend and moved back a little in 2020. But there are lots of things that could produce a trend over those elections (especially in fast-growing counties, where newcomers may have different political inclinations). Trump's presence on the ballot was a constant in 2016, 2020, and 2024, so it's not clear why it should produce a trend. You could say that he was something of an unknown quantity in 2016, so he had more effect as people got to know him better. But that wouldn't produce a difference between 2020 and 2024.
My explanation of the difference in the number of Democratic and Republican "triple-trending" counties is that it's a combination of two things: increasing association between population and Democratic share of the vote in 2016 and 2020 (it stayed about the same from 2020 to 2024) and a larger overall vote shift between 2020 and 2024 than between 2016 and 2024. The first means that a lot of small counties moved Republican in 2016 and 2020; the second means that fewer counties (of all sizes) bucked the national trend in 2024 than in 2020. If you look closely at the geographical distribution of recent shifts, there are probably some implications for the electoral college, although it's not clear which party they would favor. But they don't have any significant implications for the popular vote.