Hillary Rodham Clinton won the backing of the American Federation of Teachers Saturday, the first major labor union endorsement in the 2016 presidential race.

The national union's 36-member executive council voted overwhelmingly to support Clinton, the frontrunner for the Democratic nomination. Among them was Ted Kirsch, of Philadelphia, president of the Pennsylvania state AFT.

"You can raise a million issues with her (Clinton) – and the Republicans will – but you have to look at the big picture," Kirsch said in an interview. "She is on the right side of all these issues that are important to a strong middle class."

The endorsement gives an important labor boost to Clinton as she faces a challenge from Vermont Sen. Bernie Sanders, who has been drawing massive crowds in early caucus and primary states as he appeals to the party's liberal activists.

AFT President Randi Weingarten is a longtime Clinton friend and ally. She serves on the board of Priorities USA Action, a super PAC that is supporting the former secretary of state, U.S. senator and first lady.

Clinton advocates universal pre-kindergarten public education, supports the Common national learning benchmarks, and has promised to release a plan to limit student-loan debt for higher education. She has said that good public schools are vital to a healthy middle class.

Leaders of the AFT, which represents 1.6 million teachers, higher-education workers, nurses and other public workers, met last month with Clinton, Sanders, and former Maryland Gov. Martin O'Malley last month. No Republicans accepted the union's invitation to interview for the endorsement, officials said.

Polling of the union's membership found Clinton supported by about 70 percent of the AFT's membership who vote in Democratic primaries, Kirsch said. AFT Pennsylvania members voted at their biennial convention June 28 in Philadelphia, attended by 250 members, to recommend that the parent union endorse Clinton.

"For nearly a century, the American Federation of Teachers has worked to expand opportunity for the people and communities they serve," Clinton said in a statement. "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."

The National Education Association, the nation's largest teachers union with 3 million members, has interviewed the Democratic contenders but has not yet made an endorsement.

AFL-CIO announced Saturday that it would interview July 29 and 30 Clinton, Sanders, O'Malley and former Arkansas Gov. Mike Huckabee, who is seeking the Republican nomination, with an eye toward a possible endorsement.