WASHINGTON -- In the punch-for-punch debate over Social Security, the AARP is working hard to keep the White House on the ropes.

When President Bush arrived in Iowa Wednesday to talk up his private-accounts proposal, the senior citizens group countered him with two news conferences, the release of a national poll, full-page newspaper advertisements and commercials on radio and television.

Over the last two weeks, AARP, the nation's largest lobby, will have spent more than $5 million on ads attacking the president's Social Security plan -- nearly three times as much as all the supporters of his proposal put together.

Every state that has a swing-vote senator will have AARP forums, which have been drawing about 300 people each. And every time a member of Congress holds a town meeting, AARP volunteers are dispatched there to protest the president's plan for individual accounts.

"We're going to do this as long as it takes," said William Novelli, AARP's chief executive. "We will put just about everything we have into it."

No organization has more tools or more money to wage such a battle. So both its friends and adversaries agree: AARP holds the key to how or whether Social Security will be restructured this year. "It will be very difficult to do anything without AARP's support," said Senate Finance Committee Chairman Charles Grassley, R-Iowa. "And it would be a heck of a lot easier if they came along."

AARP's 35 million membership base is 10 times the size of the National Rifle Association's, and its $800 million budget is five times that of the U.S. Chamber of Commerce, the country's biggest business association. In number of members, AARP is surpassed only by the Roman Catholic Church.

Some polls show that a majority of voters reject Bush's plan to make investment accounts part of the retirement program, a result that can be attributed in part to AARP's persuasiveness.

"We're behind the curve right now," said Rep. Jim McCrery, R-La., a Bush ally and chairman of the House Ways and Means Committee's Social Security subcommittee. "The momentum is with the other team."

As head of that team, Novelli works 13-hour days packed with visits to Capitol Hill and the White House, where he is, by turns, berated for his position but probed to see where he might compromise.

Bush has bashed as "scare tactics" commercials by AARP and like-minded groups, and House Majority Leader Tom DeLay, R-Texas, called AARP "incredibly irresponsible" for rejecting "a solution that hasn't even been written yet." Yet behind the scenes, Novelli and his staff have been consulting with Bush aides Karl Rove and Allan Hubbard about finding common ground, and talking with congressional leaders of both parties.

For now, compromise doesn't look likely. Members of Congress and AARP officials said their meetings have involved mostly laying out conflicting views. They also say neither side is likely to budge significantly until Bush completes his 60-day campaign for his proposal at the end of next month.

But then the issue could be reopened. "I'm hopeful that maybe we can construct a plan," McCrery said. "AARP represents for us a very valuable ally if we can get it to sign on to what we want to do."

While attending a Social Security speech by the president last week, Sen. John McCain, R-Ariz., encouraged AARP to bend. "I want to say to our friends in the AARP -- and they are my friends -- come to the table with us," he said.

AARP isn't averse to changing Social Security; it merely disapproves of Bush's plan on private accounts. AARP would add such accounts as a supplement to Social Security and rejects the idea of putting the program's basic guaranteed benefit at risk with market investments. It also would be willing to fiddle with other aspects of the program to make sure it remains solvent in the long term.

AARP is an 1,800-employee association that mobilizes, better than any other group, the people who vote in the greatest numbers: senior citizens. It reaches them with the nation's largest-circulation publications, AARP the Magazine and the AARP Bulletin, which are delivered to 22 million households. It also has offices in all 50 states, a national radio show, a heavily visited Web site and a 650,000-circulation Spanish-language magazine, Segunda Juventud (Second Youth).

On top of that, AARP has taught 1.5 million members how to contact their elected representatives. Those activists receive their own publication, as well as periodic "alerts" that launch them into action from every congressional district. So far this year, the 535 members of Congress have received more than 460,000 phone calls complaining about Bush's private-account plan, according to AARP.

"When AARP has a consensus among its members, it's difficult to stop," said James Thurber, a lobbying expert at American University in Washington. "On this issue it has a consensus."

In late 2003, however, AARP shocked official Washington and much of its membership by backing Bush's drive to add a prescription-drug benefit to Medicare. This time around, AARP won't take its members' loyalty so lightly, Novelli said. Its in-house polling division will take surveys and conduct focus groups before the organization takes any new stand on Social Security changes.

That kind of work is expensive but AARP can afford it. The association took in $350 million last year from a variety of royalty-producing enterprises, including insurance, prescription drugs and mutual funds.

AARP is facing criticism of its mutual fund business because that kind of investment would probably be a feature of the accounts the president is pushing. Scudder Investments Inc., a unit of Deutsche Bank AG, handles 600,000 mutual fund accounts of AARP members, with $10.5 billion in assets. Novelli denied that there was inconsistency between AARP's resistance to Bush's plan and the fact that AARP encourages its members to invest in mutual funds, especially its own. AARP believes that Social Security's government-provided payments should remain inviolate.