Hillary Clinton picked up the endorsement of a key labor union Tuesday as she looks to lock up support from the Democratic Party base.

In a statement, the Service Employees International Union’s Mary Kay Henry, international president of the group, said, “Hillary Clinton has proven she will fight, deliver and win for working families.”

Henry added: “SEIU members and working families across America are part of a growing movement to build a better future for their families, and Hillary Clinton will support and stand with them.”

SEIU, which represents 2 million workers, is one of the nation’s largest labor unions. During Clinton’s last presidential run, in 2008, the group supported then-Illinois Sen. Barack Obama.

Clinton, the Democratic front-runner, has fared well so far in her bid for the 2016 nomination in attracting support from organized labor, even as her chief rival, Vermont Sen. Bernie Sanders, champions a message that targets working-class voters, including a proposal to raise the minimum wage to $15 per hour.

Clinton has also been endorsed by the National Education Association, the largest teachers’ union. Meanwhile, the American Postal Workers’ Union last week announced it was backing Sanders.