U.S. Sen. Bernie Sanders defended his campaign’s compensation package Friday after a Washington Post story highlighted concerns among staffers that they were not receiving the $15-an-hour wages Sanders champions on the campaign trail.

"I'm very proud to be the first presidential candidate to recognize a union and negotiate a union contract," the Vermont senator and presidential candidate told the Des Moines Register in an interview Friday. "And that contract was ratified by the employees of the campaign, and it not only provides pay of at least $15 an hour, it also provides, I think, the best health care benefits that any employer can provide for our field organizers."

He also expressed frustration that staffers had taken their complaints to the media.

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"It does bother me that people are going outside of the process and going to the media," he said. "That is really not acceptable. It is really not what labor negotiations are about, and it's improper."

He also said: "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."

Sanders said field organizers, who are the lowest-ranking members of a presidential campaign and are typically in their 20s, make $36,000 a year with 100% employer-paid health care, as well as paid vacation and sick leave.

For a staffer working 40 hours a week, that comes out to about $17 an hour. But 40-hour workweeks on presidential campaigns are rare. Sanders said the campaign will limit the number of hours staffers work to 42 or 43 each week to ensure they're making the equivalent of $15 an hour.

It’s an issue Sanders said the campaign tried to address in a proposal it offered to union leaders, though it was rejected.

Sanders employs 48 people in Iowa.

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The campaigns for U.S. Sen. Elizabeth Warren of Massachusetts and former U.S. Housing Secretary Julian Castro also have unionized. Warren, in Sioux City on Friday, said her campaign is in the middle of collective bargaining negotiations.

"I think they're going well. And we'll land in a place that's good for all of the workers," Warren told reporters. "It's powerfully important."

She did not disclose how much her campaign staff would be paid, and she declined to comment on Sanders.

"I'm not here to knock another Democrat. All I can say is we're in the middle of our negotiations and I think they're going well," Warren said.