Southern states have been described as Hillary Clinton's firewall because of Bernie Sanders' relatively poor performance with minority voters.
The online interactive tool for Reuters' 5-day rolling poll allows you to filter results according to demographic data provided by participants in the poll, essentially creating your own custom crosstabs. Let's take a look under the squiggle.
Registered voters who identify as Democrats, Nationwide:
Overall
Clinton 41% Sanders 29% Biden 16% (most recent)
White
Clinton 42% Sanders 29% Biden 15% (most recent)
Black
Clinton 56% Sanders 16% Biden 13% (most recent)
Hispanic
Clinton 43% Sanders 22% Biden 19% (average for past month)
Southeast
Clinton 56% Biden 19% Sanders 16% (most recent)
Alabama, Louisiana, Mississippi
Clinton 54% Biden 22% Sanders 15% (average for past month)
Georgia, South Carolina
Clinton 57% Biden 23% Sanders 12% (average for past month)
North Carolina, Virginia
Clinton 49% Sanders 31% Biden 11% (average for past month)
Kentucky, Tennessee, West Virginia
Clinton 41% Sanders 28% Biden 18% (average for past month)
Younger voters, age 18-29:
Overall
Sanders 58% Clinton 29% Biden 6% (most recent)
White
Sanders 61% Clinton 25% Biden 8% (average for past month)
Minorities
Sanders 44% Clinton 29% Biden 17% (average for past month)
All results are from registered voters who identify as Democrats. The most recent results are given when possible. Average results for the past month are given when the sample would be too small otherwise.
Reuters' 5-day rolling poll is conducted online by Ipsos. Ipsos has a C+ rating from 538 for online polling.
Generally, a higher percentage of participants in internet polls are familiar with Sanders than participants telephone polls, and Sanders usually performs better in internet polls. It's good to take this into account when interpreting the results.