By backing Clinton, the American Federation of Teachers, which as the nation's second-largest education union represents 1.6 million members, became the first national union to endorse a 2016 candidate.
The endorsement comes just two days before Clinton delivers a speech about the economy that labor leaders will watch closely. Some of them have already expressed public support for US Senator Bernie Sanders of Vermont, who is also seeking the nomination.
"Hillary Clinton is a tested leader who shares our values, is supported by our members, and is prepared for a tough fight," AFT President Randi Weingarten said in a statement, citing Clinton's record as a former first lady, US. senator and secretary of state.