Clinton gets major teacher's union endorsement as support for Bernie Sanders grows
Former US Secretary of State Hillary Clinton turned over paper copies of the emails in 12 cardboard boxes, and says she destroyed all the rest as they were not related to diplomatic matters but were personal (AFP Photo/Nicholas Kamm)

Democratic presidential candidate Hillary Clinton on Saturday scored a key endorsement from a large teachers union at a time when her chief rival is picking up support from organized labor.

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 U.S. 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, U.S. senator and secretary of state.

Weingarten and Clinton are longtime friends and allies. The AFT endorsed Clinton in 2007 when organized labor split between supporting her or now-President Barack Obama.

Both Sanders and Clinton are set to meet with labor leaders this week as they court a crucial base of support for Democratic candidates.

Looming over the meetings is a Pacific Rim trade deal that the Obama administration is finalizing and which has drawn vociferous criticism from labor. The issue is a difficult one for Clinton, who was Obama's secretary of state and has remained largely silent about the agreement, which Sanders has consistently opposed.

Clinton's campaign released a statement saying she was honored to receive the AFT's endorsement.

"The men and women of AFT work throughout our communities in our preschools, K-12 schools, hospitals, colleges and universities, and public agencies," Clinton said in the statement. "Their voices and the voices of all workers are essential to this country."

Clinton also has a crucial meeting at the end of the month with the executive council of the AFL-CIO, an umbrella group for

56 unions, including the AFT.

The AFL-CIO has not said whether or when it will endorse a primary candidate. In 2008, when its member unions were split between Clinton and Obama, it backed Obama only when it became clear he was going to be the Democratic nominee.

AFL-CIO President Richard Trumka has urged local and state federations to remain neutral as the process plays out.

(Reporting by Amanda Becker; Editing by Lisa Von Ahn)