“For nearly a century, the American Federation of Teachers has worked to expand opportunity for the people and communities they serve. I’m honored to have the support of AFT’s members and leaders, and proud to stand with them to unleash the potential of every American,” Clinton said in a statement.

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It is the first international labor union endorsement in the 2016 presidential race.

The AFT includes 1.6 million members and retirees around the nation.

“I know from my own family that teachers have the power to change lives. We need to make sure every child has access to a quality public education and teachers with the tools to help them succeed,” Clinton said.

The AFT ranks third in size behind the National Education Association and the Service Employees International Union.

The AFL-CIO, a federation made up of 56 smaller member unions, is not considered to be in the same category.