Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) said in an interview published Thursday that he has "serious problems" with popular mobile ride-hailing service Uber.

Speaking with Bloomberg, the Democratic presidential contender called the service “unregulated,” wading into one of the most closely watched economic debates in the nascent 2016 race.

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Workers at on-demand economy companies like Uber, Airbnb or Lyft are considered independent contractors, and therefore do not receive the benefits and protections afforded to full-time employees. Some labor advocates say the workers are being misclassified.

The company and other on-demand startups have also faced a host of other regulatory challenges, mostly at the state and local levels. It was not clear what specific issues Sanders has with the company.

The company disputed Sanders’s characterization of the company as “unregulated.” An Uber spokesman said there are 54 jurisdictions is the United States that have regulations for ride-hailing services.

Sanders’s sharp reaction to the service deviates from the playbook used by his Democratic opponents when discussing the company and others like it. They have generally treaded carefully for fear of alienating either Silicon Valley or labor-rights activists. His comments also place Sanders to the left of former Secretary of State Hillary Clinton, the Democratic front-runner.

Clinton would only go so far as to say that the growth of the on-demand economy was “raising hard questions about workplace protections and what a good job will look like in the future." She also praised the companies for “creating exciting opportunities and unleashing innovation.”

Even the campaign of former Maryland Gov. Martin O’Malley, who has positioned himself to the left of Clinton, has been quick to say that he is “optimistic about companies like Uber” while calling for new policy proposals to address questions about workers at the companies.

"Governor O'Malley has Uber on his phone, and uses it frequently,” spokeswoman Haley Morris said in a statement last month. “As companies like Uber grow and expand, we need to update our labor laws.”

Republicans have eagerly embraced the on-demand economy, and particularly the omnipresent Uber, in an effort to show that they are in touch with younger voters and the tech industry. Former Florida Gov. Jeb Bush even hailed an Uber as part of a campaign event last month.

The campaign conversation about the on-demand economy has coincided with new attention being paid to the industry by Washington policymakers.

In June, the Federal Trade Commission held a workshop on regulatory questions in the sharing economy, which drew thousands of comments. And Sen. Mark Warner (D-Va.) has said he will roll out a set of policy proposals meant to design a safety net for workers at companies like Uber.

The courts are also considering similar questions. On Thursday, a federal judge in California heard arguments for and against allowing a lawsuit filed by Uber drivers against the company to proceed as a class-action suit. Lyft and other firms face similar legal actions — which threaten to strike at the core of their business model.

—This story was updated at 7:35 p.m.