The senator, propelled by a groundswell of support, is also gaining ground on Clinton in polls emerging from across the early voting states.
The difference between Bernie Sanders and the other candidates, in this respect, is that Bernie has flat out refused to accept super PAC support. Most of his money is coming directly from people that support him:
"Aides to Mr. Sanders said that since April 30, nearly 400,000 contributions had come into the campaign, with 99 percent of those donations in increments of $250 or less."
Whichever source we choose to believe for the number of donors, you also can't deny his history of accepting contributions either. He has quite the clean record:
I have a ton of respect for Bernie Sanders, and I know that if he wins the nomination and the election it will truly be because of popular support rather than clever campaigning and rich contributors.
EDIT: Also for the curious, check out Hillary Clinton's contributors and compare them to Bernie Sanders' donors.