The head of the American Federation of Government Employees said the union was confident they "can count on Hillary Clinton to help us" do their jobs, in a post on their website
"Secretary Clinton shares AFGE's vision for a strong and vibrant government workforce that has the necessary tools and support needed to deliver vital programs and services to the American public," said J. David Cox, the union president.
AFGE represents 670,000 workers in the federal government, including those that work for the District of Columbia government.
The endorsement -- which as first reported by The Associated Press
- is the 18th national union endorsement for Clinton and will help the Democratic front-runner's campaign argue that she has solidified the national union movement around her candidacy, not that of Vermont Sen. Bernie Sanders.
Clinton accepted the endorsement in a statement on Thursday, pledging, as president, to "always stand up for workers' rights, fight to keep workers safe on the job, and protect retirement security, including defined benefit plans and Social Security."
The federal employee union came to its decision by sending questionnaires to all announced presidential candidates and conducted surveys of its membership. According to the union, "Clinton's support exceeded that of the next closest candidate's by a 2-1 margin."
The AFGE National Executive Council, the organizing body for the union, then voted this week to endorse Clinton.
The endorsement is yet another union blow to Sanders, who has worked hard to court union support, including marching on multiple picket lines in an attempt to show his commitment to the effort. While that has yet to pay off for the independent senator, the Sanders campaign has routinely said after each national endorsement for Clinton that while union leadership is with Clinton, the people on the ground are with Sanders.
AFGE's endorsement is not binding for its local chapters and members. But in the post on its website, the union urged members to "close ranks and stand by her in what will no doubt be a long fight against a determined anti-government, anti-worker onslaught."