... The tipping point came when [AARP policy chief] Mr. Rother learned that the White House was seriously considering tackling the program during Mr. Obama's first term.

On AARP's Policy Council, a group of volunteers who make policy recommendations to the AARP board, Mr. Rother faced intense opposition from members who see their role as protecting low-income seniors. He also faced resistance from the board.

His argument: Tax increases wouldn't be enough to make the program solvent. The leading proposal for raising taxes—increasing the amount of income subject to payroll taxes, the central financing mechanism for the program—would fill less than half the hole. Moreover, Republicans were not going to accept a plan that didn't include benefit cuts. The idea that both tax increases and benefit cuts were needed dovetailed closely with plans put forward by several separate commissions in Washington seeking to ease the U.S.'s long-term fiscal woes.

"There was good, healthy discussion," said John Penn, chairman of Intek Plastics Inc., a member of AARP's board. "Healthy tension usually results in better answers, but sometimes it's painful in the process."

When Mr. Obama considered making a Social Security proposal early this year, Mr. Rother indicated he would be supportive, said two people familiar with the matter. But the White House opted to hold off.

AARP is steadfast in its opposition to tackling Social Security as part of a grand budget deal—the kind currently being discussed by a group of lawmakers led by Vice President Joe Biden—saying Social Security didn't cause the current deficit and shouldn't be used to fix the problem. An AARP news release last month urged seniors to lobby their members of Congress against "political deals that cut their hard-earned benefits."

It is also determined that any deal should be bipartisan, a stance influenced by backlash to the group's support for Mr. Obama's health-care law. AARP's endorsement made a big difference in Democrats' ability to secure passage, given that the bill included a half-trillion dollars in cuts to Medicare, the federal health-care plan for seniors.

The group lost about 300,000 members as a result and came under attack from some Republicans who accused AARP of supporting the law because the group, which helps sell insurance products, benefited financially.

On the Social Security question, the organization will have to sell its position to members—which likely will be a major challenge. A February Wall Street Journal/NBC News poll found 84% of Americans age 65 and up opposed benefit cuts. Internal AARP polling shows similar resistance.