US presidential candidate Bernie Sanders’ campaign has been accused of illegal interrogation and retaliation against staffers during a bitter union dispute.

A formal complaint was made to the National Labor Relations Board this week with five allegations of misconduct, including interrogation, illegally sacking an employee, repudiation, modification of contract and illegal discipline.

Bernie Sanders has been locked in a union dispute with his campaign workers. (AAP)

The complaint was made by a staffer in Bettendorf, a town in the crucial early voting state of Iowa.

The board received the complaint on July 19, and posted a list of allegations on its site on Monday.

It comes after a bitter union dispute between Senator Sanders and the staff on his presidential campaign.

Bernie Sanders campaigning in Iowa, the crucial midwestern state that votes before any other place. (AAP)

The self-described Democratic Socialist from Vermont has made raising the minimum wage to US$15 ($21) an hour a centrepiece of his campaign.

The candidate's presidential staff are largely on a salary which would be equivalent to that hourly wage, provided they work 40 hours a week.

But the staffers are arguing that because of copious amounts of overtime, they are paid less than Senator Sanders' suggested minimum.

Wage earners who made less than US$35,000 a year are entitled to overtime pay in the United States.

Bernie Sanders waves to a Donald Trump supporter during a parade in Iowa. (AAP)

Staffers on Senator Sanders' campaign are paid $36,000 a year.

Hours ago it was announced his campaign had made a deal with employees, bumping their pay up to US$42,000 a year.

Senator Sanders is the first presidential candidate whose staff have unionised.

Who is Bernie Sanders?

Bernie Sanders is a senator from the northeastern state of Vermont.

Though an independent, he caucuses with the Democrats, and is now running for president with that party.

Bernie Sanders is considered a frontrunner of Democratic candidates running for President. (AAP)

As the left-most candidate in the race, the 77-year-old has advocated an entirely public health care system, higher taxes on the middle and upper class, and a vow to break up the big banks.

As the main rival to Hillary Clinton in the 2016 primary, Senator Sanders captured the popular zeitgeist among younger voters.

Bernie Sanders is running for president in the Democratic primary against candidates like Joe Biden and Kamala Harris. (AAP)