PITTSBURGH — President Barack Obama told the AFL-CIO today his administration has "stopped our economic free fall" and will work hard to seal a recovery while revitalizing America's middle class.

Seeking to shore up a key part of his political base, Obama told the labor federation's convention that the administration is a fierce supporter of workers' rights and defender of unions. He also challenged union members to stand with him on the hard-fought push for an overhaul of the health care system.

"One of the fundamental reasons I ran for president was to stand up for working families," Obama said on a day designed to appeal to politically active union members.

"When our middle class succeeds, that's when the United States of America succeeds. That's what we're fighting for," he said.

In a message tailored to workers, Obama praised organized labor figures gathered in Pittsburgh for their role in creating a middle class and for propelling the economy forward during last century. He said the same groups must help push the economy ahead now.

The appearance was part of Obama's aggressive effort to sell his health care overhaul following his speech last week to a joint session of Congress and a slew of interviews and appearances the White House has arranged for him in the coming days.