Democratic presidential candidate and Sen. Bernie Sanders (I–Vt.) has been one of the nation’s leading advocates pushing to raise the federal minimum wage to $15 per hour, but campaign staffers say the senator isn’t even paying his own staff that rate.

According to internal documents reviewed by the Washington Post, the campaign has been rocked by an ongoing labor dispute since the beginning of May, with unionized staffers demanding the senator treat employees the same way Sanders has been pushing large corporations to—giving workers higher wages.

A draft letter, obtained by the Post, was addressed to campaign manager Faiz Shakir from union members. It said that low-level staffers “cannot be expected to build the largest grassroots organizing program in American history while making 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 authors wrote.

According to the Washington Post, field organizers are paid an annual salary of $36,000 and not paid by hours worked. Regional field directors for the campaign are paid $48,000 and statewide department directors are paid $90,000 a year.

The Post reports that the letter from staff argued field organizers are working at least 60 hours a week and therefore making an hourly rate of $13 per hour.

“Many field staffers are barely managing to survive financially, which is severely impacting our team’s productivity and morale. Some field organizers have already left the campaign as a result,” the letter continued.

A draft proposal from staffers urges the campaign to bump up the pay for field organizers to $46,800 a year and to $62,400 for the regional field directors, the Post reported.

Sanders has proudly noted that his presidential campaign was the first to unionize its staff. Other campaigns that have since unionized their staffs include those of Sen. Elizabeth Warren (D–Mass.) and former Housing and Urban Development Secretary Julian Castro.