WASHINGTON - President Obama's offer to limit the growth of Social Security benefits would cost the average retiree less than $50 in the first year. But the cuts would grow over time, and that has advocates for seniors worried that Democrats in Congress will break their promise to shield the massive retirement and disability program from cuts in deficit-reduction talks.

Both Senate Majority Leader Harry Reid (D., Nev.) and House Minority Leader Nancy Pelosi (D., Calif.) pledged not to touch Social Security as part of negotiations to avoid the year-end fiscal cliff. Reid, however, is backpedaling now that Obama and House Speaker John A. Boehner (R., Ohio) have agreed to a new measure of inflation that would reduce annual cost-of-living adjustments, or COLAs, for Social Security and other government programs.

Obama and Boehner continue to haggle over how much to raise taxes and cut spending, but both have agreed to the new inflation measure formula, making it increasingly likely the proposal would be part of an eventual deal. Boehner proposed the change earlier this month in talks with Obama, and the president included it in a counteroffer this week.

White House press secretary Jay Carney called the new inflation measure a technical adjustment designed to make inflation estimates more accurate. Obama did not directly address the issue at a White House news conference Wednesday. But, he said, a deal will require difficult choices.

"What I've said is that in order to arrive at a compromise, I am prepared to do some very tough things, some things that some Democrats don't want to see and probably there are a few Republicans who don't want to see either," Obama said.