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A major teachers’ union on Saturday voted to make an early endorsement of Hillary Rodham Clinton, a lift to her presidential campaign as she tries to fend off a stronger-than-expected challenge from the left.

The endorsement from the American Federation of Teachers was not a surprise. The group is led by Randi Weingarten, a longtime ally of Mrs. Clinton, and it backed her in her losing primary battle against Barack Obama in 2008.

But the union is giving her its support again at an opportune moment for Mrs. Clinton, just before her first major speech on the economy, scheduled for Monday, which is seen as an attempt, in part, to neutralize the criticism leveled at her by her leading challenger, Senator Bernie Sanders of Vermont. Mr. Sanders has surged in the polls by appealing to the populist anger of many Democrats over economic issues.

Education policy remains one of the few areas of unsettled debate within the Democratic Party. President Obama’s education agenda has often infuriated the teachers’ unions, and last year, the head of the National Education Association, another union representing teachers, called for the secretary of education, Arne Duncan, to be fired.

Advocates of changes to the system, including some major Democratic donors, consider Mr. Obama an ally and have been pressuring Mrs. Clinton to adopt Mr. Obama’s posture. But in meetings with the A.F.T. last month, Mrs. Clinton said that teachers shouldn’t be the “scapegoat” for society’s ills.

After that meeting — members also heard from Mr. Sanders and Martin O’Malley, the former Maryland governor — Ms. Weingarten praised Mrs. Clinton’s performance and detailed responses to members’ concerns.

The timing of the endorsement risks angering other union leaders, particularly Richard Trumka, the president of the A.F.L.-C.I.O. Mr. Trumka has called on Mrs. Clinton to embrace labor’s economic agenda, and has been privately urging individual unions to hold off any endorsements.

But in an interview, Ms. Weingarten said the membership overwhelmingly supported Mrs. Clinton, something which became clear in two town halls and in two separate member polls the group took in the last six weeks.

As a senator, Ms. Weingarten said, Mrs. Clinton was given a 100 percent rating from the teachers’ union on her voting record. She said she had repeatedly heard from the union’s members that they believed Mrs. Clinton would “make the country a fairer place for working people.”

And Mrs. Clinton has deep ties to the teachers’ union, beyond the endorsement it made in her 2008 campaign. She worked frequently with Ms. Weingarten when Ms. Weingarten led the union in New York City. In March, just before Mrs. Clinton made her candidacy official, she and Ms. Weingarten appeared together on a panel in Washington to discuss expanding opportunities in urban areas sponsored by the Center for American Progress, the liberal group that has advised the Clinton campaign as it settles on an economic framework.

In addition, former President Bill Clinton’s chief of staff, Tina Flournoy, is a former A.F.T. official.

In addition to discussing women and family issues on Monday, Mrs. Clinton is expected to support Mr. Obama’s plans to extend overtime pay to workers and strengthen collective bargaining. She will also emphasize policies that would help the largely female members of the A.F.T., including pushing for paid family leave and affordable childcare, according to aides.