Survey of peoples second favorite candidate broken down by their first choice. I would have expected second choices to follow ideological lines. So Warren supporters would switch their support to Sanders if Warren dropped out because he's also on the socialist side of the spectrum.
Instead second choices break down by gender/race. People whose first choice is an old white man have a second choice of an old white man. Voters his first choice is a woman have a second choice for a woman. Voters whose first choice is a black man have a second choice of a black man.
Or concretely the second choice for Warren voters is Harris. The second choice for Harris voters is Warren. The second choice for Booker voters is Harris - not Biden or another moderate democrat. The second choice for Biden voters is Sanders and for Sanders voters its Biden. The only exception seems to be Buttigieg voters whose second choice is Harris.
Original poll is here https://morningconsult.com/2020-democratic-primary/ but I didn't link to it because I think it'll go away