Multiple reports out today suggest that Dem leaders in the House and Senate are edging towards supporting Chained CPI for Social Security as part of the “grand bargain” Obama wants to replace the sequester with — and that’s already sparking sharp pushback from Congressional liberals.

“Why are we doing this?” Dem Rep. Keith Ellison, a co-chair of the Congressional Progressive Caucus, said to me in an interview today. Asked which is worse, continued sequestration or a grand bargain that cuts entitlement benefits, Ellison said: “It’s like saying, `Which of your kids do you want to sacrifice to the monster?’ Neither one.”

Ellison is backed up by over 100 other House Dems who have pledged to fight any cuts to retirement benefits, including Chained CPI, a way of indexing Social Security benefits to inflation that amounts to a real benefits cut.

At a presser today, Nancy Pelosi signaled openness to Chained CPI, saying: “If we can demonstrate that it doesn’t hurt the poor and the very elderly, then let’s take a look at it. Because compared to what? Compared to Republicans saying Medicare should wither on the vine?” Meanwhile, the Post quoted administration officials claiming both Pelosi and Harry Reid are on board with the grand bargain and are ready to rally rank and file Dems to support politically difficult cuts to retirement programs in exchange for real tax increases.

“Leader Pelosi has always encouraged members to offer their own sincerely held views,” Ellison told me. “My sincerely held view is that Chained CPI is a benefit cut for people who have very little. An overwhelming number of people who are on Social Security have fixed incomes. We have a lot of people across America who agree. Most of our caucus is opposed to this.”

When it comes down to it, isn’t the choice just between extended sequestration and some kind of deal to replace it, and if so, which is worse? Is this the choice liberals face? I put the question to Ellison, and he rejected the framing, arguing that being drawn into it is to already cede ground to Republicans.

“Once we do that we’re already in the territory of bargaining away Chained CPI,” Ellison said. “We’re already saying we’re open to negotiating on Chained CPI. And we’re not.” Senator Bernie Sanders has similarly insisted that liberals must not allow the choice to be framed this way, and has instead called on the White House and Dem leaders to try to leverage public opinion to force Republicans to accept a long term deal that includes increased revenues and cuts spending judiciously without targeting entitlement benefits.

Ellison pointed out that Republicans aren’t as quick as Dems to signal a willingness to trade away core priorities at the outset. “Republicans don’t do that,” he said.

The sharp language from Ellison, Sanders and other liberals shows that Obama and Dem leaders will face a stiff headwind from the left if they stray too far on to “grand bargain” territory. The endgame here, however, remains murky. If Obama and Dem leaders do reach some kind of deal with Republicans in the Senate, some liberals might support it in the end if the President asks them to, just as liberals have previously proven willing to give away core priorities to advance his agenda. Or a deal might simply pass without liberals. Wherever this is headed, for progressives who want to make their opposition to any “grand bargain” benefits cuts known, the time is now.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.