While all eyes are on House Republicans ahead of Thursday evening's planned vote on Speaker John Boehner's (R-Ohio) 'Plan B,' an afternoon news conference by several of the chamber's most liberal members served as a reminder that Democrats have concerns of their own about a potential "fiscal cliff" compromise.

The half-dozen members of the Congressional Progressive Caucus gathered outside of the House side of the Capitol to renew their call for President Obama to exclude from a potential deal any changes to the definition of inflation used in calculating annual Social Security cost-of-living increases.

Such changes, known as "chained CPI," would constitute a benefit cut for the most vulnerable Americans, the Democrats argued.

"You can't tell me that chained CPI isn't a real cut," Rep. Jan Schakowsky (D-Ill.) said.

Rep. Donna Edwards (D-Md.) called on Obama to "keep his promise to leave Social Security out of the equation."

Obama had put chained CPI on the table during last year's debt ceiling negotiations, and liberal Democrats also opposed it then. Proponents contend that chained CPI is a more accurate measure of inflation that would constitute a cut in the rate of growth for benefits rather than a reduction to benefits themselves, generating as much as $100 billion in deficit savings over the next decade.

Opponents -- including the country's largest federal workers union, which issued a statement this week opposing chained CPI -- maintain that Social Security should be off limits in the "fiscal cliff" talks and that the change in the inflation index would represent a significant blow to groups including seniors, veterans and the disabled.

“It’s a trophy for some of my colleagues in Congress; it’s a trophy,” Rep. Raul Grijalva (D-Ariz.) said Thursday of the calls from some Republicans to include chained CPI in any fiscal bargain.

Rep. Keith Ellison (D-Minn.) made an allusion to “a candidate in the last election” who spoke about receiving Social Security survivor benefits. Rep. Paul Ryan (R-Wis.) often noted on the campaign trail that he received such benefits as a young man after his father died.

Retiring Rep. Barney Frank (D-Mass.), who also spoke at the event, disputed the notion that some Democrats are opposing chained CPI for political reasons, noting that he won't be up for reelection and that his position is one based on "my moral sense."

Asked whether the inclusion of chained CPI would be a deal breaker for them, however, none of the assembled lawmakers would say.

Frank said only that "that broader package ain't gonna be no Christmas present" and that chained CPI would be a "pretty heavy burden" for liberal Democrats.