A major chunk of Bernie Sanders' record fundraising is coming from an unlikely source: people without jobs.

Sanders raised more than $16 million in 2015 from more than 235,000 people who identified themselves as either unemployed or retired, according to filings with the Federal Election Commission. Almost all of it has come through ActBlue, an online platform for making political donations to Democrats.

Sanders has raised more money than any candidate—Republican or Democrat—except for Hillary Clinton. And he's done it while railing against the billionaire class and their influence on politics. Nearly three-quarters of the $73 million that the self-declared democratic socialist has raised has come from donations of $200 or less. In contrast, of the $108.9 million Clinton has raised, 17 percent has come from small donors.

Sanders has gained traction in the polls with his message that the U.S. economy has been tilted to favor the wealthy; that free trade agreements and high health care costs have imperiled the middle class; and that high tuition costs and student loan debt are a barrier to economic mobility.  

"He's got a lot of support and obviously it's from people who don't have a lot of money," Richard Seltzer, professor of political science at Howard University, said. "In Iowa and New Hampshire he got 80 to 85 percent of young voters. A lot of them are people who are unemployed or underemployed or not fully employed."

With about 2.4 million subscribers, ActBlue allows people to contribute to their favorite Democratic candidate or cause with a click of the mouse or tap on a smartphone. The $54.7 million that Sanders raised via the platform shows that it can provide financial power that rivals super-PACs, which can take unlimited contributions from donors. More than a quarter of Sanders' ActBlue haul has come from those who said they weren't employed. The Sanders campaign didn't respond to requests for comment.

In the first 24 hours after Sanders won the New Hampshire primary, ActBlue processed $5.2 million in contributions for the campaign. In the first 15 minutes after Sanders asked for donations in his victory speech, the platform processed 26,000 contributions—a record for the site.

"Instead of having a billionaire write a single check, we have millions of small donors who contribute,” said Erin Hill, executive director of ActBlue. The platform taps into a broad base of modest contributors. As Sanders did on the night of the New Hampshire primary, candidates can return to the same pool of donors for additional funds because they're still below the legal limit for campaign contributions.

Hill stresses that ActBlue is more than just an e-commerce portal for campaign contributions. "It's also an engagement tool,” she said. “If you give $5 to a campaign, you're going to follow that campaign and you're going to vote.”

ActBlue, which operates at the federal level as a PAC, collects and reports information on all who contribute to candidates using its platform, including those who give as little as one dollar, giving insight into the kinds of small donors funding Sanders' campaign.

In 2014, in order to boost fundraising efforts, Act Blue abandoned earlier distinctions on donors' occupations, lumping into a single category "not working", two earlier separate identifiers which were "unemployed" and "retired." That, they said, helped boost the funds raised by 5 percent but it also confounded any deeper analysis on the number of donors in each cohort or how much the retired gave compared with the unemployed.

Among Sanders' other supporters who used ActBlue, individuals describing themselves as self-employed contributed $2.3 million. Employees of Google contributed $95,000, the top company represented, followed by the University of California with $40,000 and Microsoft with $27,000. Among industry groupings, those employed by educational establishments gave more than $800,000, while government workers—federal, state and county—contributed $264,000.

ActBlue donors have directed a little more than $35,000 to Hillary Clinton, though her campaign is relying on other technology for its online donors. Clinton campaign spokesman Josh Schwerin said that more than 750,000 people have donated to Clinton's campaign, and the majority of donations has been online.

“Like President Obama in previous elections, we have focused on building a balanced operation that can provide the resources we need to run a strong campaign that can win a long and competitive primary,” Schwerin said.

Donors in blue states—those won by President Barack Obama in 2012—gave about $20 million to Sanders, compared to only $5 million from those in red states, with California and New York in the top two places. Sanders' home state of Vermont, whose donors ranked dead last in 2014 in giving by state, came in tenth.