

Supporters of Sen. Elizabeth Warren (D-Mass.) at a caucus Monday in Des Moines. (Charlie Neibergall/AP)

What makes the Iowa caucuses unique, besides serving as an inadvertent software beta-testing laboratory, is that they include voters’ second picks in their calculus.

You’ll remember how the caucuses work. Those in attendance show up and stand with other supporters of the same candidate. If there aren’t enough supporters of that candidate to hit a required threshold, those supporters need to lure other voters to their fold or go support someone else. Usually, it’s the latter option that occurs, and caucus-goers settle on a candidate other than the one they first supported.

It can be hard to tell how that realignment worked across all of the precincts that held caucuses Monday. In many precincts, supporters of multiple candidates were forced to change their minds and pick new candidates, but they often didn’t all choose the same one. We know how many caucus-goers shifted away from former vice president Joe Biden among the precincts for which we have data, for example — about 2,400 — but we don’t necessarily know where they went.

Except in places where we do. There are some precincts where Biden was the only candidate to lose supporters, according to the data from the Iowa Democratic Party. In those places, we can see which candidates benefited. If we apply that same calculus for all of the candidates in the mix in Iowa — looking at precincts where only one candidate lost supporters — we can get an admittedly limited picture of how support shifted.

It looks like this, with the candidates losing supporters listed vertically and those gaining support listed from left to right.

The biggest overall shift was from businessman Andrew Yang to former South Bend, Ind., mayor Pete Buttigieg. Forty-seven Yang supporters — not many! — identified Buttigieg as their second choice. More on those two in a second.

There are interesting flows among the front-runners, too. About half of those forced to a second choice after initially supporting Sen. Bernie Sanders (I-Vt.) moved to supporting Sen. Elizabeth Warren (D-Mass.). But Warren supporters pushed to another choice were about as likely to back Buttigieg as Sanders. Sen. Amy Klobuchar (D-Minn.) also did decently among those who had originally backed Warren. Klobuchar’s supporters went mostly to Biden and Buttigieg. Biden’s also went mostly to Buttigieg.

You see the pattern there: Over and over, Buttigieg was the second choice of choice, so to speak. Tallying losses and gains across the field, Buttigieg gained the most support as a second pick, followed by Klobuchar. Yang saw the biggest erosion of support — and didn’t gain much support as a second pick.

Candidate Lost Gained Biden 68 83 Buttigieg 30 179 Gabbard 5 0 Klobuchar 82 107 Sanders 74 98 Steyer 32 5 Warren 120 78 Yang 194 12

The data above, while representing only a sliver of what happened in Iowa, seems to shed quite a bit of light. Sanders had the most initial support in the caucuses, according to the limited data that has been released. But Buttigieg leads in the calculus of how support translates into delegates. Part of that is a function of where his support was distributed. Part of it, too, is that he gained ground between the first choice and the final realignment. As of writing, Sanders went from 24.4 percent support to 26.2 percent support between the two tallies. Buttigieg jumped from 21.4 percent to 25.2 percent.

This doesn’t do Buttigieg much good in New Hampshire, where the primary is a straightforward tally of votes. But it does suggest that Buttigieg may have an edge moving forward if he remains the No. 2 choice for voters, as their first choices might give up on seeking the nomination.

Yang, on the other hand, might be justified in being a bit nervous.