Bernie Sanders' Democratic presidential campaign has raised $15 million since launching on April, with a large number of small cash donors.

Ninety nine percent of the 400,000 contributions from 250,000 donors gave $250 or less, with an average donation of $33.51. Pushing against "the billionaires" and the "1 percent" has been a hallmark of Sanders' campaign. The Vermont senator refuses to form a super PAC.

Most donations came online through his campaign website, The numbers are found in a preliminary report by his campaign, but will be finalized by the Federal Election Commission's July 15 deadline.

The announcement from the campaign comes after Sanders held the biggest campaign rally of the 2016 season thus far, with 10,000 coming out to see him in Madison, Wisconsin.

"I am more than aware that my opponents will be able to outspend us. But we are going to win this election," Sanders told the cheering crowd at the Veterans Memorial Coliseum. "They may have the money but we have the people. And when the people stand together, we can win."

A Quinnipiac poll released Thursday has Sanders gaining ground on the Democratic front-runner Hillary Clinton in Iowa. Clinton still 52 percent to 33 percent among likely caucus-goers, but in the beginning of May just 15 supported Sanders with 60 saying they'd support Clinton.