Sanders, campaigning in South Carolina on Friday, said he was proud of the distinction.
"We cannot just support unions with words, we must back it up with actions," he tweeted. "On this campaign and when we are in the White House, we are going make it easier for people to join unions, not harder."
The United Food & Commercial Workers Local 400 confirmed that Sanders' campaign workers were the first to win union representation.
"We expect this will mean pay parity and transparency on the campaign, with no gender bias or harassment, and equal treatment for every worker, whether they're in Washington, D.C., Iowa, New Hampshire or anywhere else," UFCW Local 400 President Mark P. Federici said in a news release.
Working on political campaigns at any level often involves grueling hours and low pay. And recently, workers on some campaigns have been pushing to unionize.
Earlier this year, Sanders apologized to female staffers on his 2016 presidential campaign who said they had experienced sexual harassment from male staffers. The New York Times had reported allegations of unwanted sexual advances, as well as pay inequity.
At that time, Sanders said his 2018 re-election campaign to the Senate representing Vermont established "some of the strongest sexual harassment policies in the country," including requiring training for all employees and providing staffers with a way to report harassment to an independent firm.
UFCW said Sanders' campaign had agreed to use "card check" to determine whether most workers wanted a union. In card check, union organizers ask workers to sign cards affirming that they support a union. If a majority of workers sign them, the union presents those cards to the employer so that the employer will recognize the union.
The union said that by Friday, a majority of the campaign's 44 bargaining-unit-eligible employees had signed union cards. Sanders' aides said on a recent conference call that the campaign has roughly 70 paid staffers. The union says that all campaign employees below the level of deputy director will be represented by the union and that negotiations for a collective bargaining agreement will begin "as soon as possible."
Federici called the unionization a breakthrough that he hoped would serve as a model for other campaigns.
"While political campaigns aren't the easiest work environment, every worker has the right to respect and dignity," he said. "And when candidates who claim to support the labor movement practice what they preach, that sends a powerful message that, if elected, they will deliver on their promises to strengthen union rights and level the playing field between workers and employers."