Workers for Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE's (I-Vt.) 2020 presidential campaign are reportedly pushing for higher wages as the candidate urges support for a nationwide $15 an hour minimum wage.

Campaign field employees are asking for salaries they say would be equal to the $15-an-hour wage Sanders has championed, The Washington Post reported Thursday, citing internal documents.

The documents reviewed by the newspaper reportedly show that the unresolved dispute between the unionized workers and management has been going on since at least May.

A draft letter members of the union planned to send to campaign manager Faiz Shakir said that field organizers “cannot be expected to build the largest grassroots organizing program in American history while making poverty wages," according to the Post. "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."

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Organizers worked at least 60 hours per week, resulting in pay of less than $13 per hour, the letter reportedly estimated.

“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 reportedly said.

The Post reported that field organizers' annual salary was $36,000, while regional field directors received $48,000 and statewide department directors were given $90,000.

In May, Shakir suggested increasing field organizers' pay to $42,000, the Post reported, citing an email to staffers. The union's draft letter reportedly suggested he wanted to create six-day workweeks.

Shakir in a statement to The Hill Friday touted the contract with the union.

"We are very proud to be the first presidential campaign in American history to recognize a union and negotiate a contract," he said. "That contract was ratified by the employees of the campaign and not only provides pay of at least $15 an hour, it provides platinum level health care ... paid vacation, sick leave and other benefits."

He also said the campaign and union have been discussing pay structures "to ensure that anyone working beyond 40 hours a week will still continue to receive $15 an hour."

"We look forward to continuing those discussions and obviously are disappointed that some individuals decided to damage the integrity of these efforts before they were concluded," he added. "As these discussions continue, we are limiting hours of a few employees to ensure that no one is receiving less than $15 for any hours worked."

The United Food & Commercial Workers Local 400, of which the Sanders's campaign employees are part, told The Hill in a statement that it "cannot comment on specific, ongoing internal processes between our members and their employer, particularly without the consent of our members."

"We look forward to continuing to work closely with our members and the management of the Bernie 2020 campaign to ensure all workers have dignity and respect in the workplace," the union added.

Sanders earlier this year introduced a bill to raise the federal minimum wage to $15 an hour. He is among more than two dozen candidates for the Democratic Party's 2020 presidential nomination.