Biden is the most likely second choice among Amy Klobuchar’s backers, in particular. In a Biden-win scenario, they and others below the viability line would shift to him, pushing him slightly over the top.

Biden’s other advantage is that his support is spread out fairly evenly across the state. To win the Iowa caucuses, a candidate essentially has to win delegates from every part of the state. Biden’s support would add up piece-by-piece to a narrow statewide edge for him.

In that case, Biden would get to 27% and 13 national delegates out of Iowa; Sanders, with 26%, would receive 10 delegates; Buttigieg, at 23%, would receive 10 delegates; and Warren, 18% and 8 delegates. But we should also note that Biden’s vulnerabilities include having supporters who haven’t been enthusiastic about him and have been considering other candidates in our recent polling. That could open the door for Sanders, among others.