WASHINGTON — AARP, the nation's largest seniors organization, is coming out strongly against President Bush's plan to allow private individual accounts within Social Security.

In the newsletter it sends to its 35 million members, the group says Bush's plan would damage the most successful government program in history and abdicate on a promise made to future retirees.

"Taking some of the money that workers pay into the system and diverting it into newly created private accounts would weaken Social Security and put benefits for future generations at risk," says the December issue of the AARP Bulletin, which went to members this weekend.

The group's position is consistent with its longstanding opposition to privatizing the nation's retirement program. But it was the strongest statement yet to its members and comes as Congress and the White House gear up to tackle the issue.

Opposition from one of the most formidable advocacy groups in Washington could make it much harder for Bush to make headway in setting up private accounts. White House spokeswoman Pam Stevens declined to comment directly on AARP's position but said, "The Social Security system is unsustainable," and the president "believes in solving problems, not passing them on to future generations."

Last year, the AARP gave crucial backing to Bush's plan to create a controversial prescription drug benefit under Medicare, a move that drew heavy fire from members and Democratic allies in Congress. That in turn led to speculation that it might hedge on its opposition to the push for private Social Security accounts.

"This is not a new position, but the fact that they are stating it in a very clear manner to their millions of members has to be considered quite significant," said Robert Greenstein of the Center on Budget and Policy Priorities, an anti-poverty group.

"This is our signature issue," AARP's head of policy and strategy, John Rother, said in an interview.

Social Security relies on payroll taxes deducted from workers' checks to pay benefits. Excess payroll taxes are held in a trust fund for future benefit payments. When baby boomers begin to retire, costs will rise and payouts will begin to exceed revenues in 2019, the Congressional Budget Office projects.

In its message to members, the group acknowledges that the system, last overhauled in 1983, will need to be shored up again in the near future.

While Bush has promised to make revamping Social Security a top priority for his second term, the administration has offered no specific plans. Opponents say when Bush does flesh out a proposal for private accounts, it will become clear that it will entail benefit cuts, additional risk for retirees and a ballooning national debt.