Analysis of polling data shows Biden entering hurts Clinton, not Bernie [View all]
This article compares the poll result of polls including Biden vs those without Biden. While true that this likely is not completely what one would have seen had Biden actually announced he was in the race, it is one of the few attempts to use polling data to investigate who would be most impacted.
As the article explains, there are various theories that depend on what you believe motivates supporters of Sanders, Clinton or Biden. However, plotting the poll estimates shows that Clinton is very affected and Bernie is not really impacted at all. (They estimate a 6 point loss from HRC's numbers due to Biden's inclusion in the polls. I am skeptical of this estimate because there really are only a small number of data points available to fit the Biden included line. ) The author himself includes many caveats.
Still, it is an interesting way of looking at the poll results. http://www.washingtonpost.com/blogs/monkey-cage/wp/2015/09/06/polls-show-a-biden-presidential-run-would-hurt-clinton-not-sanders/