Presidential candidate Sen. Bernie Sanders on Friday defended the salaries his campaign pays its workers and expressed frustration that the details of a tense dispute between unionized staffers and upper management became public.

In a statement to The Washington Post, Sanders said he was “very proud” that his was the first presidential campaign to unionize workers.

“We have a historic contract agreement that provides unprecedented protections and benefits,” he said in the statement, which was released by his campaign. “Through that framework, we are committed to addressing concerns in good faith through the bargaining process.”

The Washington Post reported Thursday that unionized Sanders field staff are embroiled in a standoff with upper management over their salaries. The Post obtained emails, instant messages and other documents that revealed details of the negotiations between union members and Sanders campaign manager Faiz Shakir.

Those materials showed that salaried campaign staff complained that because of the extensive hours they worked, they made less than the equivalent of $15 per hour — the rate Sanders has demanded the nation adopt as a minimum wage. They have cited the senator’s words and principles to Shakir in making their case for more money. The documents were provided to The Post on the condition of anonymity because of the sensitivity of the private talks.

The Post reported Thursday that it was unclear when or whether Sanders had been made aware of the situation. His statement gave no indication.

Sanders has made standing up for workers a central theme of his presidential campaigns, both in 2016 and this year. His defense of the working class has been a signature element of his effort, as he has argued that politicians from both major parties work to benefit corporations and the wealthy.

In an interview with the Des Moines Register published before his statement, Sanders said he was frustrated that information about the labor difficulties had been publicized.

“It does bother me that people are going outside of the process and going to the media,” Sanders told the Register. “That is really not acceptable. It is really not what labor negotiations are about, and it’s improper.”

He added, “We are disappointed that some individuals have decided to damage the integrity of these efforts. We are involved in negotiations. And some are individuals that have decided to damage the integrity of that process before they were concluded.”

According to the Register, Sanders said the campaign will limit the number of hours staffers work to 42 or 43 hours per week so that they make the equivalent of $15 an hour.

The Sanders campaign’s field organizers, who are on the front lines of the campaign’s contact efforts, make $36,000 annually. A draft of a letter union members were preparing this week estimated that field organizers were working a minimum of 60 hours per week, dropping their average hourly pay to less than $13. The campaign compensates its interns on an hourly basis, paying them $15 to $20 an hour.

On May 17, Shakir recommended raising the pay for Sanders field organizers to $42,000 and changing the workweek specifications, according to an email he later wrote to staff. The union draft indicated he was seeking to extend the workweek to six days.

The union rejected his offer, because the raise would have elevated field staff to a pay level responsible for paying more of their own health-care costs, according to the draft proposal the union was preparing this week.

On Friday, other 2020 campaigns offered more details of their compensation structure.

Speaking to reporters Friday morning after a town hall meeting in Orange City, Iowa, Sen. Elizabeth Warren (D-Mass.) said it is “of course” important to pay her campaign staff a $15 per hour minimum wage. “I’m very proud of the fact that our staff decided to unionize,” she said.

Warren said her campaign is in the midst of a collective-bargaining process she called “powerfully important.” She declined to criticize Sanders.

“Look, I’m not here to knock another Democrat,” Warren said. “All I can say is, we’re in the middle of our negotiations, and I think they’re going pretty good and well.”

Her field organizers make $3,500 per month, the campaign said, which works out to $42,000 a year. The campaign would not immediately say how long its organizers’ workweeks typically span.

The campaign of Pete Buttigieg, the mayor of South Bend, Ind., said its organizers are paid $3,500 a month and work 50-hour weeks, a rate of about $16 per hour.

Former Texas congressman Beto O’Rourke’s campaign said base pay for all staff is $50,400 annually.

The Sanders campaign pays all health-care premiums for salaried employees making $36,000 or less per year. The Warren and Buttigieg campaigns said their field organizers also have their coverage paid in full.

Annie Linskey, Jenna Johnson and Amy B Wang contributed to this report.