WASHINGTON, D.C. — The longtime head of a union that represents thousands of school bus drivers across the U.S. said last week that he is retiring.

Andy Stern has served as president of Service Employees International Union (SEIU) for the past 14 years. He joined the union in 1972 as a member of Local 668 in Pennsylvania.

As Stern steps down, SEIU Secretary-Treasurer Anna Burger will assume the role of interim president until a successor is voted on.

SEIU spokesperson Diane Minor told SBF that the union represents an estimated 18,000 school bus drivers in eight states and hundreds of school districts. The largest numbers are in California, Minnesota and Washington state.

Stern is credited with doubling the union’s membership during his tenure, to 2.2 million members, and with helping elect Barack Obama to president and helping pass healthcare reform. SEIU made an early endorsement of Obama, and the union was reportedly the largest contributor to his election efforts.

Stern has often been a target of criticism in the editorial pages of the Wall Street Journal and from Fox News commentators, and he is seen by some in the labor community as a divisive figure.

The National Union of Healthcare Workers, which broke away from SEIU, is fighting the larger union for the allegiance of tens of thousands of healthcare employees in California, according to the Los Angeles Times.