Washington - US Democratic presidential front-runner Hillary Clinton, who has been vying with challenger Bernie Sanders for the support of organised labour, scored a win on Saturday with an endorsement from the National Education Association, a huge teachers union.
The move makes the NEA, the largest US labour union with 3 million members, the latest in a string of unions to support Clinton, including the American Federation of Teachers and the machinists' union.
Clinton's lead in opinion polls has been narrowing against Sanders, a Vermont senator who has rallied progressives with his pledges to tackle income inequality and rein in Wall Street.
NEA President Lily Eskelsen Garcia praised Clinton as a strong leader who would work for students, teachers and families "because she understands the road to a stronger US economy starts in America's public schools".
Clinton issued a statement saying she was honoured to get the NEA endorsement.
"I've stood with educators throughout my career ...," she said. "As president, I will fight to defend workers' right to organise and unions' right to bargain collectively, and I will ensure that teachers always have a voice and a seat at the table in making decisions that impact their work."
Clinton is trying to win over labour - often a key source of volunteers and funds for Democrats - in the effort to build a broad coalition within her party and avoid a potentially damaging, drawn-out primary fight.
The winner of the primary contest will face the Republican nominee in the November 2016 election.
Clinton's campaign has struggled recently following a steady stream of news about her use of a private e-mail server and e-mail address while secretary of state.
A rolling five-day poll by Reuters/Ipsos dated September 29 found Clinton's support within her party at 44%, compared to 28% for Sanders. As recently as August she had 56% support for the party's nomination.
Labour leaders around the country are pressing Clinton on issues ranging from the minimum wage to international trade. One flashpoint is the Trans-Pacific Partnership, a proposed free-trade agreement backed by President Barack Obama but opposed by unions, which see it as bad for US jobs and wages.
Clinton has remained neutral on the deal, saying a final agreement must protect American workers.
She was secretary of state during Obama's first term and was part of the administration's push to strengthen ties with Asia.