WASHINGTON – Democratic Illinois Senate hopeful Tammy Duckworth picks up major backing on Monday when the SEIU Illinois State Council endorses her over rivals Andrea Zopp and Napoleon Harris.

The endorsement comes after the union, which represents 150,000 workers in Illinois, screened Duckworth and Zopp. Harris did not reach out to the SEIU for an endorsement, a union spokesman told the Sun-Times.

Duckworth, a two-term House member, and Zopp, an attorney, and Harris, a state senator, are competing in the March primary for the nomination to run against Sen. Mark Kirk, R-Ill. Kirk is well positioned to win his GOP primary.

The SEIU endorsement is particularly important because the union has a cadre of experienced political activists who know how to canvass, phone bank, get out the vote and provide other related boots-on-the-ground assistance. The SEIU also can bolster Duckworth through assisting third-party groups and encouraging members to make direct donations to the Duckworth campaign.

SEIU Illinois State Council President Tom Balanoff said in a statement that Duckworth “is not only a war hero but has a proven record of supporting issues crucial to working families, such as ensuring equal pay for women, expanding access to maternity leave and child care, raising the minimum wage, and passing comprehensive immigration reform.”

“We look forward to continuing to work with her in fighting for a fair economy that works for everyone,” Balanoff said.

Balanoff and other SEIU Illinois State Council leaders will appear with Duckworth at a press conference on Monday morning at union headquarters in downtown Chicago.

A more immediate concern for Harris is qualifying to get on the March ballot since Zopp allies filed challenges to his nominating petitions. Zopp and Harris have in common an African-American political base. Duckworth is the frontrunner – made stronger with the SEIU endorsement – and central to Zopp’s strategy is consolidating African-American support. Zopp’s path is easier if Harris is knocked out of the contest.

The SEIU Illinois represents public employees, social service, medical and other workers.