The largest federal employee union will work to defeat President Bush in November after endorsing presumptive Democratic nominee John F. Kerry last week.

The American Federation of Government Employees (AFGE), which has 210,000 dues-paying members, said the Massachusetts senator would help turn back the Republican's efforts to revamp federal pay and personnel systems and open up more federal work to private contractors. Kerry, who has voted with the AFGE 88 percent of the time during his Senate career, also opposes private accounts in Social Security, favors spending more on veterans' health care and would strengthen Medicare and collective bargaining rights, said John Gage, the AFGE president.

"Senator Kerry is against this dismantlement of the fed government," Gage said in a conference call with reporters during a gathering Friday of 600 union delegates in Pittsburgh. "People believe in what they've done in their careers. . . . They've spent their lives to make these programs work and they feel that at the top there is a concerted effort to make these programs not work."

Allison Dobson, a Kerry spokeswoman, said the campaign is "thrilled" to have the union's support. Terry Holt, a Bush campaign spokesman, called the endorsement "utterly predictable," given the AFGE's frequent criticism of White House labor policies.

"We would still ask the rank-and-file members to make a clear choice between the two candidates when they go to vote on Election Day," Holt said. "We think that the president's policies will appeal to every American."

In an April speech, Kerry pledged to cut 100,000 federal contractor jobs, cap federal travel budgets and streamline agencies to rein in administrative costs by 5 percent.

The AFGE, affiliated with the AFL-CIO, ranks seventh among public-sector unions in political contributions this election cycle, according to the nonpartisan Center for Responsive Politics. The union has donated more than $490,000 to candidates, with 84 percent going to Democrats.

The AFGE joins several other unions, collectively representing more than 400,000 employees, that have said they will back Kerry. They include the National Treasury Employees Union (NTEU), the International Federation of Professional and Technical Engineers (IFPTE) and the American Federation of State, County and Municipal Employees.

The NTEU, which represents 150,000 workers in 29 agencies, endorsed Kerry on Feb. 2. Colleen M. Kelley, the union's president, said Kerry had voted for federal pay raises and to increase funding for federal agencies, while opposing Bush administration efforts to change overtime laws and contract out jobs to private firms.

The AFGE and NTEU endorsed Democrat Al Gore over Bush in the 2000 election. The IFPTE, with 40,000 members in the Defense Department and NASA, typically makes no endorsement, but it broke with that tradition because "this administration has been so anti-labor, particularly when it comes to federal employees," said Matthew S. Biggs, an IFPTE spokesman.

Gage said 5,000 AFGE members have volunteered to work on behalf of the Kerry campaign. The union plans to focus on states whose electoral votes appear to be up for grabs, such as Pennsylvania, West Virginia, Ohio and New Mexico. The AFGE also plans to spend more than $1 million on campaign advertising. The union's goal is for at least 73 percent of its members to vote for Kerry, up from 68 percent for Gore four years ago.

Researcher Lucy Shackelford contributed to this report.