As the Democratic field for president thins out, most supporters of the dropouts will pick another candidate. That prospect raises the question of whether these newly available voters will flock to the front-runners or boost the chances of one of the also-rans. To gain insight into this question, the latest YouGov polls asked respondents likely to vote in Democratic primaries to name both their first and second choices for the nomination. In the most recent survey, Joe Biden leads on first-choice ballots, followed by Elizabeth Warren, Bernie Sanders, Pete Buttigieg and Kamala Harris. So what happens to the horse race when we add in the second choices of potential voters?

The short answer is that Joe Biden hemorrhages support, as the table below shows.

Biden, who has a narrow lead over Warren on first-choice ballots, does not draw as many second choices as Warren and falls almost five percentage points behind after combining the first two columns. Sanders also leads Biden on the second-choice ballot, 10.3% to 8.5%, which brings Sanders to within 10 points of Biden (though still more than 14 points behind Warren).

Buttigieg, with 7.5% total on both ballots, reaches 15% , while Kamala Harris is just below him, at 12.7%. Significantly, no one in the rest of the field reaches even 4% across the two ballots (note that the “total” column adds up to more than 100% because we are essentially “double counting” each voter — a person gets to vote once for their top candidate and a second time for their backup).

The second choice for supporters of the two most liberal candidates is typically (and unsurprisingly) the other liberal. Pluralities of Warren and Sanders supporters list the other as their second choices, with 24% of Warren supporters choosing Sanders and 36% of Sanders supporters choosing Warren (nearly 35% of each candidate’s voters indicated they had no second choice).

This pattern suggests that either Warren or Sanders would receive a significant boost were the other to drop out. Biden’s voters, by contrast, spread their second ballot between Sanders, Warren, and Buttigieg (13%, 16%, and 8%, respectively), with a remarkable 47% saying they had no second choice. Buttigieg supporters went primarily to Warren (21%), Biden (19%), and Harris (16%), while Harris supporters went for Warren (28%) and Buttigieg (17%). Interestingly, first-ballot Harris and Buttigieg voters showed little appetite for Sanders, with fewer than 5% of each listing him as their second choice.

The upshot of these survey results is that the order in which the candidates leave the race matters. If, let’s say, Sanders were to drop out due to health reasons, Warren would be the most prominent beneficiary, as she is the second choice of over a third of Sanders’ voters (compared to only 13% for Biden). If, for some unlikely reason, Warren were to leave the race, Sanders would be helped far more than Biden. If Biden were to bow out, there would be no clear-cut winner, as Sanders, Warren, and Buttigieg would split his votes relatively evenly.

If the second tier of Buttigieg and Harris go away, Warren would be the clear winner.

Unsurprisingly, these results do not bode well for the third-tier candidates in the Democrats’ field. None of the other candidates receives even 3% of the second-choice votes, and less than three months remain before the Iowa caucuses. Moreover, neither Thanksgiving nor Christmas provide occasion for voters to pay close attention to politics. Thus, there is little time for these candidates to emerge.

With their national polling numbers weak, Iowa becomes particularly important for this group, as an unexpectedly strong showing offers the potential for a boost going forward. However, the evidence is once again stacked against the underdogs. The latest YouGov/CBS battleground poll in Iowa gave the top three candidates 65%, and Buttigieg and Harris 19%, leaving all the others to fight over a mere 16% of the vote.

Likewise, the latest RealClearPolitics Iowa polling average has the top five candidates with a little over three-quarters of the vote. Indeed, the major movement in Iowa has been towards Buttigieg, who has moved into second place as Biden and Harris begin to fade there. Of the third-tier candidates, only Amy Klobuchar has reached 4%, a number that still leaves her well off the pace.

After Iowa comes New Hampshire, where the current polling averages are similar. There, Biden, Sanders, and Warren collectively account for 66% of the vote, with Buttigieg and Harris capturing another 13%. The remaining 15 or so candidates split the other 21%, leaving none with any significant support. In short, as candidates begin to drop out — and as Iowa and New Hampshire further cull the field — the beneficiaries will likely be the top-tier candidates rather than the bottom-feeders.

David Brady is a professor of political science at Stanford University and the Davies Family Senior Fellow at the Hoover Institution.