Clinton speaks during a campaign stop at the Iowa City Public Librar on Tuesday. (Charlie Neibergall/AP)

The American Federation of Teachers endorsed Hillary Clinton for the Democratic presidential nomination on Saturday, the first national union to back a candidate for the 2016 primary.

The endorsement was not a surprise to close observers - the AFT had supported Clinton in 2008 instead of Barack Obama - but the early timing may be designed to give Clinton a boost against her surging rival, Sen. Bernie Sanders (I-Vt.).

"In vision, in experience and in leadership, Hillary Clinton is the champion of working families need in the White House," AFT president and longtime Clinton ally Randi Weingarten said.

In a statement released by the union, Clinton said she was "honored" by the nod. "I know from my own family that teachers have the power to change lives, " she said. "We need to make sure every child has access for a quality public education and to the teachers with the tools to help them succeed."

Weingarten and Clinton have been friends since their days in New York, when Weingarten led the city's teachers union and Clinton ran successfully for the U.S. Senate. Weingarten sits on the board of the pro-Clinton PAC Priorities USA.

The 1.6 million-member AFT, along with its sister union, the 3 million-member National Education Association, have been under siege from both elements within the Democratic and Republican parties.

The unions have been fighting the expansion of public charter schools, which are largely not unionized, as well as teacher evaluations based on test scores and challenges to tenure and other workplace protections.

The unions have both been critical of many of the education policies of the Obama administration, saying they have led to a "blame the teacher" culture. They argue that evaluating teachers based on student test scores does not recognize the complexities of teaching students who often come from impoverished homes or struggle with disabilities and language barriers that affect their achievement.

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Clinton, Sanders and former Maryland Gov. Martin O'Malley all met with both the AFT and NEA leadership last month in an effort to win their backing.

Clinton struck a particularly sympathetic tone in her meeting at the NEA, telling the union that people are "dead wrong to make teachers scapegoats for all of society's problems."

Clinton's relations with teachers unions didn't begin as smoothly, when she first entered public life. As first lady of Arkansas in 1982, Clinton pushed to broaden course offerings in public schools, smaller class sizes and institute competency testing for teachers - an idea that provoked a fierce pushback from the union.

But as a U.S. senator and first lady, Hillary Clinton promoted policies much more friendly to the teachers unions, including expanding preschool and after school programs. As a presidential candidate in 2008, she opposed merit pay for teachers, another stance in line with the unions.