Hillary Clinton will propose ending the "Cadillac tax" on some employer-provided health care plans.
President Barack Obama's Affordable Care Act penalizes companies whose premiums are higher than $10,200 a year for individuals and $27,500 for families, which encourages companies to drop the plans which unions and workers like.
A senior official of the American Federation of Teachers said union President Randi Weingarten was contacted by Clinton campaign aides in recent days to inform her of the move.
The union in July endorsed Clinton, who indicated at the time she had concerns about the tax. The tax doesn't go into effect until 2018 and has bi-partisan opposition in Congress.
Clinton's move could help draw support for her campaign from other unions that have been holding off making an endorsement.