WASHINGTON (AP) — Vermont Sen. Bernie Sanders has won the endorsement of the 200,000-member American Postal Workers Union, the largest labor union to back his Democratic presidential campaign.

The union's decision gives Sanders a boost heading into the second Democratic debate, scheduled for Iowa this weekend. It also comes as the Vermont senator has sought to stop a string of labor endorsements to Democratic front-runner Hillary Rodham Clinton.

Clinton has locked down the support of several large unions, including the National Education Association and the American Federation of State, County and Municipal Employees.

The postal workers' union says Sanders has a long history of supporting postal employees and has worked to keep post offices open in small towns and fought attempts to privatize the mail service.