Hillary Clinton addresses the annual meeting of the U.S. Conference of Mayors in San Francisco, June 20, 2015. The American Federation of Teachers endorsed Clinton for the Democratic nomination Saturday. Reuters/Stephen Lam

The executive council of the American Federation of Teachers voted Saturday to endorse Hillary Clinton for the Democratic nomination for president of the United States, a press release from the union said. AFT previously backed Clinton in her losing battle against Barack Obama in 2008.

With unexpected difficulty for Clinton early in the campaign against Bernie Sanders, the early endorsement offers an opportunity for the former secretary of state to regain momentum. AFT is the first national union to endorse a candidate in the 2016 Democratic presidential contest.

"In vision, in experience and in leadership, Hillary Clinton is the champion working families need in the White House," AFT President Randi Weingarten said in a statement released to the press. "Hillary Clinton is a tested leader who shares our values, is supported by our members and is prepared for a tough fight on behalf of students, families and communities."

Clinton has had a long friendship with Weingarten, the New York Times reported, and while the endorsement was expected, many see education as a divisive issue in the Democratic party, with unions and wealthy donors taking opposite sides on charter schools, teacher tenure and testing.

"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 responding to the endorsement. "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 tools to help them succeed."

The endorsement comes two days before Clinton will give one of her first major speeches on the economy at the New School in New York. She is expected to highlight the stagnant wages of the American middle class.