"It is about joining forces to build a more united labor movement that can fight back against the corporate and political onslaught facing our members each and every day," Hansen said.
The UFCW has 1.3 million members who work in the retail, meat-packing and food-processing industries. It will become the AFL-CIO's largest private sector union, giving the federation about 13.3 million members.
Two other unions that left the AFL-CIO in 2005 have also come back in recent years, including the Laborers and the union of hotel, restaurant and clothing workers known as UNITE HERE.
The announcement Thursday comes as AFL-CIO President Richard Trumka pushes for major changes that could see the federation expand to include environmental and civil rights groups like the Sierra Club and the NAACP. Trumka says organized labor needs to broaden its reach to workers beyond those with collective bargaining agreements.
The move by the UFCW leaves just three unions remaining in Change to Win the Service Employees International Union, the Teamsters, and the United Farm Workers. While Change to Win has been more aggressive in helping its unions wage new organizing campaigns, it never emerged as a true rival to the AFL-CIO, which has been the face of organized labor for more than a half-century. Former SEIU leader Andy Stern who spearheaded the 2005 split left the union in 2010 and is no longer a force in labor.
"It puts the another stake through the heart of Change to Win, which was a mistake driven by the personality of Andy Stern," said Nelson Lichtenstein, a labor historian at the University of California, Santa Barbara, and director of the Center for the Study of Work, Labor and Democracy. "It's better to have the house of labor united than divided."
Paco Fabian, a spokesman for Change to Win, said the UFCW would continue to work with the federation's Strategic Organizing Center.
"We'll continue to do some of the cutting edge organizing we've been doing," Fabian said. "It's a difficult moment for labor with density overall pretty low and we need to figure out how to do things differently in order to reverse that."
Despite the split, unions in the two federations have worked together and pooled resources to fight efforts in Wisconsin, Ohio and other states where officials have sought to take away collective bargaining for public employees and to curb other union rights.
The decision to rejoin the AFL-CIO also could give the UFCW more resources in its longtime quest to organize workers at Wal-Mart, the nation's largest retailer. Wal-Mart has vigorously resisted efforts to unionize its employees, even closing one of its Canadian stores after workers there voted to form a union. Two years ago, the UFCW helped create OUR Walmart, a nonunion group of Wal-Mart employees that has pushed for better pay and working conditions.