We can try to answer that question by looking at how polling last fall looked with and without Biden in the race. The Post and ABC News polled in September and October, and we can look at the results of those two polls through two lenses: How support broke down among all three candidates and how it broke down if we reallocate Biden’s support to voters’ second choices. What’s interesting about these two polls is that they reflect two very different moments in the race. The September poll was a particularly bad one for Clinton, putting her at just over 40 percent — one of her lowest levels of support of the campaign. By October, she’d rebounded somewhat.
In that September poll, Clinton earned the support of 42 percent of the Democrats and Democratic leaners that were surveyed. Take Biden out, though, and her support jumped to 56 percent. Which means that Biden not being in the race gave Clinton a 14-point boost — and that one-third, 33 percent, of her support in a one-on-one contest with Sanders came from people who would otherwise have backed Biden. For Sanders, the numbers were 24 percent with Biden and 28 percent without — so 17 percent of his one-on-one support came from Biden.
We can do a similar calculation for all of the demographic groups with large enough sample sizes to be significant. And we can do it for both September and October, to get a sense of how much support Clinton got thanks to possible Biden backers during a bad month and how much she got during a good month.