He has been attracting much larger crowds in Iowa, too. If Sanders were to sweep those two states, it could damage Hillary’s campaign beyond repair. Remember that this poll was completed before it was revealed that Hillary lied about having Top Secret intelligence reports in her emails that were kept on a private server, which is against federal law and State Dept. protocol.
The FBI is now in possession of the thumb drives that were in the possession of her lawyer, David Kendall. Hillary has announced that her server is being handed over to the Justice Department.
Although no one expects Loretta Lynch to allow criminal charges to be filed against Clinton, it is the damage to her already shaky numbers on honesty and trustworthiness.