The most recent Wall Street Journal poll highlights Bernie Sanders biggest obstacle – gaining the vote of minorities.
As stated in the WSJ:
Despite Mrs. Clinton’s recent slide in the polls, Democratic primary voters are more open to supporting her than any of her rivals—or potential rivals in the case of Mr. Biden. But Mrs. Clinton lost ground to Mr. Sanders among just about every slice of the Democratic electorate, and the poll results suggest Mr. Biden would draw support almost entirely from her, rather than from Mr. Sanders.
. . .
But Mrs. Clinton boasts one powerful advantage that poses a major impediment to Mr. Sanders: Her 76%-to-16% lead over Mr. Sanders among voters from minority groups, a critical constituency that tends to hold outsize sway in Democratic nominating contests. That advantage narrows when Mr. Biden’s name is added to the mix, with 59% of nonwhite voters preferring Mrs. Clinton and 26% favoring the vice president.