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Still little evidence that Warren is taking voters from Sanders

About two months ago, I looked at some polling data to see what was going on with the Sanders / Warren crossover vote. My main finding is that nobody really knows, since we aren't collecting the specific kind of data we'd need to find this out. But given a few assumptions, I was also able to arrive at a second conclusion: their crossover vote was mostly a wash.

In the two months since then, things have changed, however - Warren's numbers have jumped, propelling her past Sanders outside of early states, and even into the lead according to some polls. With that in mind, I decided to run the numbers again. Here's what it looks like now:

Despite all of the horserace movement, the percentage of supporters each candidate has who would support their rival as a second choice has barely budged: 1st Sanders / 2nd Warren voters are hovering around 27%, and 1st Warren / 2nd Sanders are fluctuating around 23%.

So where are Warren's new voters coming from? I think that another chart makes it pretty clear:

As Warren has surged, all of the significant movement within the coalitions of the top three candidates has been between her and Biden. The number of Biden voters who prefer Warren as a second choice has jumped 11% since August, and the number of Warren voters who prefer Biden as their second has jumped 7%. A simple explanation for this is that Warren has become more appealing to Biden voters, to the point that some of them have migrated over to her camp. Meanwhile, all of the other numbers have fluctuated around within the margins of error.

Again, this kind of analysis makes some significant assumptions about what compositional changes in second-choice coalitions actually mean; we just don't have the data, at this point, to understand these crossover dynamics with any certainty.
Nevertheless, this explanation has more basis in the data than a lot of the pure speculation that's going around, and it happens to correspond well with a pretty plausible narrative of the primaries. Elizabeth Warren is surging, and the most likely explanation for this is that she is drawing from the large pool of voters orbiting around Biden, and perhaps from undecided and lower-ranked candidates (like Kamala Harris). There remains little evidence, however, that she is drawing much support from Bernie Sanders, though this could change in the months ahead.