We’ve reached the part of the Democratic presidential primary where voters’ second choices become almost as important as their first.

Former vice president Joe Biden leads the Democratic field with the support of about 28 percent of the Democratic electorate — meaning if he ends up as the nominee, as many as 7 in 10 Democrats didn’t support the party’s eventual candidate at this point. For every other candidate, that figure is even higher. Any of them, Biden included, needs to be the second choice of as many voters as possible as their first-pick candidates drop out. More immediately, they need to be voters’ second choices in Iowa, given how that state’s caucuses work.

On Thursday, though, we got a glimpse at another fascinating bit of second-choice shifting: how Democrats’ views shifted from 2016 to 2020.

Pew Research Center has been interviewing a consistent panel of voters since the 2016 nominating contest. That allows them to compare views of the current field with the preferences of the same voters four years ago.

Those who supported Hillary Clinton four years ago are most likely to now support Biden, rather unsurprisingly. So is the group of voters who at different times during the 2016 campaign indicated they supported either Clinton or Sen. Bernie Sanders (I-Vt.), or who had no opinion. About 17 percent of that group now supports Sanders.

Interestingly, while Sanders is the most commonly supported candidate among his 2016 backers, a quarter of his support from four years ago now goes to Sen. Elizabeth Warren (D-Mass.). Warren also scoops up a bit over a tenth of Clinton’s 2016 support.

A Twitter user wondered why Sanders was nonetheless in the top tier of the field. RealClearPolitics’s average of polls has Sanders in second place, with support of 22.5 percent of the electorate. But that’s far less than the support he had four years ago at this point, when he was supported by more than 37 percent of the electorate. Back then, there were only two significant candidates. Now, there are more — and Sanders’s strong core base of support keeps him in the hunt even though his support is lower than it was.

It’s not unexpected Sanders and Warren share an overlapping base. On Tuesday, we looked at polling from Quinnipiac University showing whom supporters of individual candidates identified as their second choices. The graphic below shows how that support shifts, with arrows scaled to the density of support that would move between candidates.

You’ll notice that the arrows between Sanders and Warren are among the thickest in the graphic, indicating the largest number of supporters of each identifying the other as their second choice.

Pew asked a question about how second choices shifted between 2020 candidates, as well. We combined those numbers with Quinnipiac’s to create a tool showing how each poll suggests support would shift from each candidate.

Second choices according to Quinnipiac and Pew polling.

Pick a first-choice candidate:

You’ll notice, again, that Warren-Sanders overlap. Biden’s support, however, is more broadly distributed, with the candidate getting more than about a fifth of his support as a second choice.

This interactive is likely to change once Iowa votes. But if you are a caucus-goer in Iowa on Monday looking to game out how the realignments will break down, let the tool above be your guide.