Andy Stern, president of the 2.2-million member Service Employees International Union, plans to resign, a senior union official told employees in an e-mail on Monday.
The message from the official, Diane Sosne, was obtained by The Associated Press. She is a member of the union's board and president of an SEIU local based in Seattle.
SEIU spokeswoman Michelle Ringuette said only that Stern plans to address "speculation" that he would leave his post later this week, at the close of the union's executive committee meeting.
Stern, 59, is one of President Barack Obama's closest political allies and a major force in Democratic politics. His union spent about $60 million to help Obama win the presidency and millions more on House and Senate races around the country.
News of his pending resignation surprised many SEIU officials reached late Monday, who had not yet officially been informed of the move. Stern's current term is scheduled to end in 2012.
Stern saw the passage of health care reform this year, one of his longtime goals, but failed to win legislation making it easier to organize unions.
Since 1996, when he became president of the SEIU, Stern has led the union to grow faster than any other, adding over 800,000 workers in the last decade.
He famously led his union and several others to break away from the AFL-CIO - the nation's largest labor federation - to form the rival Change to Win federation. He complained that the older federation focused too much on political campaigns and not enough on recruiting new members.
Stern has swelled his union's ranks with aggressive, hard-driving tactics, but his topdown management style has rankled many who complain that Stern has ignored the interests of members.