Senator Bernie Sanders’s presidential campaign staff will be represented by a union, his campaign announced on Friday, saying it would be the first time a major-party candidate had a unionized work force.

The employees will be represented by a unit of the United Food and Commercial Workers union. They voted to unionize through what is known as a card-check system: A majority of employees signed forms indicating their interest. Mr. Sanders has previously called for that method to be used at all companies, instead of traditional elections.

“Bernie Sanders is the most pro-union candidate in the field, he’ll be the most pro-union president in the White House, and we’re honored that his campaign will be the first to have a unionized work force,” his campaign manager, Faiz Shakir, said in a statement.

One other Democratic candidate, Julián Castro, said in January that he would be supportive if his employees chose to unionize.