As some lawmakers begin to quietly discuss possible gives and takes for a “grand bargain” on the current budget impasse, progressive House members and their allies gathered Thursday morning, hoping to send a clear message: There’s no room at the negotiating table to consider cuts to Social Security.

The rally was intended as a “pre-rejection” — in the words of Social Security Works Executive Director Alex Lawson — of any serious attempt to enact a change to the way inflation is calculated, known as “chained CPI.” Using a new formula for inflation would reduce future cost-of-living increases for Social Security.

Though President Barack Obama has said he won’t negotiate over “the full faith and credit of the United States,” House Republican leaders appear to be trying to extract concessions for a deal to wrap government spending and an extension of the debt ceiling into one package.

And because Obama has in the past made references to an openness to look at the chained CPI as a means of deficit reduction, some people are starting to get nervous.

“It worries me,” said Rep. Jerrold Nadler, D-N.Y., adding that a default on the nation’s debt would make the stock market crash of 1929 “look like a picnic.”

“We are organizing inside and out” to rally opposition against a chained CPI or social security benefit cuts in any “grand bargain,” added Rep. Jan Schakowsky, D-Ill.

The Congressional Progressive Caucus spearheaded the event, which concluded with lawmakers and several dozen participants forming a “human chain.” CPC Co-Chairman Keith Ellison, D-Minn., led a few versus of the old spiritual “This Little Light of Mine.”

“Part of what we’re doing here is we’re trying to set the terms of the debate,” Ellison said in an interview with CQ Roll Call. “What I want to make sure is that, as some people are having backdoor deals, and say, ‘Oh, we have a way to get out of this,’ they don’t say, ‘Oh, we’ll just toss them the seniors.'”

Ellison said he suspected that Democratic leadership would back members of the Progressive Caucus in rejecting the use of a chained CPI for deficit reduction. Even Obama, Ellison added, would not likely cave to those demands, as he has not mentioned the chained CPI in the context of the government shutdown.

“I don’t think he’s willing to give into extortion,” Ellison said.

But the Minnesota Democrat signaled he was willing to make some sacrifices: at one point he had pledged to whip the CPC against the $986 billion level included in the House’s CR, but Ellison now says he would swallow that bitter pill just to turn the government lights back on.