While Hillary Clinton has made a political career with the support of the financial sector, socialist Sen. Bernie Sanders (I., Vt.) has relied on labor.

A review of campaign fundraising records conducted by the Labor Union Report found that all of Bernie Sanders’ top donors come from Big Labor, which could complicate Hillary Clinton’s tack to the left to woo union support.

Nine of Clinton’s ten most generous donors are investment banks, white collar law firms, or companies, such as Cablevision and Time Warner, that have a history of union disputes. Sanders, on the other hand, has the backing of the working man.

"Sanders enjoys a lot of support from labor unions. Though he has had donors from other sectors, according to the Center for Responsive Politics, all of Bernie Sanders top donors over his career are union political action committees (PACs)," the site said (emphasis in the original).

Peter List, a former labor organizer who runs the Labor Union Report, said this could expose a key weakness for Clinton as she tries to stave off Sanders running to her left.

"There’s not a union among" her top donors, List said. "If union members turn out to vote in the Democratic primaries, Bernie Sanders—and not Hillary Clinton—may very well be the Democrats’ next nominee."

Sanders has been making inroads amongst Democratic voters, despite its disadvantage in fundraising and staff compared to the Clintons. The Clinton campaign has established the largest New Hampshire campaign team in history, according to the Washington Post. But that has not stopped Sanders from rising in the polls. He trails Clinton by only 8 points in the second primary state, the first poll in which the former secretary of state led by less than double digits.

The Clinton camp has moved to the left in an effort to win support from unions that spurned her in the 2008 primary against President Barack Obama. She endorsed the Service Employees International Union-backed Fight for 15 movement at two SEIU events in May and June. That movement seeks to extract $15 minimum starting wages at fast food restaurants, such as McDonalds, and has also laid the groundwork for eventually unionizing franchise chains.

Clinton’s embrace of the effort has alienated the business community. The International Franchise Association, which represents the interests of more than 800,000 franchise operations in the country, said Clinton was putting her political interests ahead of those of small business owners.

"We truly hope that she will listen to the concerns of America’s local franchise small business owners and their employees, not just the paid, union-backed protesters trying to carry-out a blatant, politically-motivated agenda on behalf of organized labor," IFA president Steve Caldeira said in a statement.

Clinton’s leftward drift stands at odds with the people and organizations that have bankrolled her political career, as well as her family foundation. The Clinton Foundation has collected more than $10 million from union targets Walmart, McDonalds, and Yum Brands, which owns Taco Bell and Kentucky Fried Chicken, according to foundation disclosures.

Neither the Clinton nor the Sanders campaigns returned a request for comment.