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Vermont Senator Bernie Sanders has led the public charge for a universal $15 minimum wage, but some of his presidential campaign employees are complaining privately that they’re not being paid much.

Field organizers for Bernie’s campaign say they work 60 hours per week and make a salary of $36,000 annually, according to ongoing wage negotiations reported by the Washington Post Thursday. That’s an average of $13 per hour.

The Post obtained copies of letters between Sanders staff and Bernie’s campaign manager Faiz Shakir outlining the employees’ demands. Field organizers — who were the first of any major presidential candidate to unionize — are reportedly preparing to send Shakir a letter that refers to their salary as “poverty wages.”

“Given our campaign’s commitment to fighting for a living wage of at least $15.00 an hour, we believe it is only fair that the campaign would carry through this commitment to its own field team,” the letter reads, according to the Post.

The Democratic-controlled the House of Representatives this week passed a measure to bring the national minimum wage to $15 per hour, though its chances of winning in the Senate are slim. Sanders introduced a bill in the Senate earlier this year that would raise the federal minimum wage to $15.

“If you work 40 hours a week, you should not live in poverty,” Sanders said in January.

Field organizers have traditionally earned low salaries, with average wages at around $37,000 annually, Glassdoor estimates show. Sanders campaign staff have requested a salary of $46,800 for field organizers and $62,400 for regional field directors.

Shakir reportedly suggested raising organizers’ pay to $42,000, but the union rejected that offer and argued that it would force employees to pay more in health care costs. The campaign currently pays all healthcare premiums for salaried employees making $36,000 or less.

The letter field organizers have reportedly drafted also makes reference to the effect of their salaries on morale. “Many field staffers are barely managing to survive financially, which is severely impacting our team’s productivity and morale,” it reads, according to the Post. “Some field organizers have already left the campaign as a result.”

Sanders’ staff have engaged in ongoing wage negotiations with Shakir since at least May, but they remain unresolved. Shakir did not immediately respond to a request for comment from VICE News but touted Bernie’s “pro-worker and pro-labor” stance in a statement to the Post.

“We know our campaign offers wages and benefits competitive with other campaigns, as is shown by the latest fundraising reports,” Shakir told the Post on Thursday. “Bernie Sanders is the most pro-worker and pro-labor candidate running for president. We have tremendous staff who are working hard. Bernie and I both strongly believe in the sanctity of the collective bargaining process and we will not deviate from our commitment to it.”