WASHINGTON — Donna Mae Litowitz, a Miami Beach retiree, likes Senator Bernie Sanders of Vermont so much that three months ago she sent his presidential campaign $10,000. His campaign sent back all but $2,700 because it was more than he was allowed to take under federal election law, but she wishes he had kept it all.

“I like what Sanders stands for, and he says what needs to be said,” said Ms. Litowitz, who gave money in 2008 to Senator Barack Obama’s presidential campaign. “And I don’t like Hillary Clinton.”

In an election dominated by million-dollar donations to “super PACs,” Ms. Litowitz qualifies in Mr. Sanders’s insurgent campaign as a big donor. Unlike almost all of the other major Democratic and Republican candidates this year, Mr. Sanders has refused to accept support from super PACs, relying instead on supporters like Ms. Litowitz as well as tens of thousands of small donors giving as little as $5 or $10.

The average donation, according to campaign officials, is $31.30.

The result is a campaign built on populist issues like income inequality that appears to be drawing even more rank-and-file support than Mr. Obama did in 2008, when he used a network of smaller donors to win the White House.