House liberals are launching an organizing drive inside the Democatic caucus, in an effort to line up Democrats and get them to commit to opposing any final deficit deal that contains any cuts to entitlements benefits, according to a letter I’ve obtained.

The two co-chairs of the Congressional Progressive Caucus have sent the letter to Nancy Pelosi, pledging to stand behind her insistence that the final deal contain no benefits cuts to Social Security, Medicare, or Medicaid.

The letter will be used next week to gather signatures from House Dems, in an effort to maximize for Democrats by developing a unified front behind a refusal to support any cuts to those programs that reduce benefits or eligibility. The letter — which was sent my way by a source and is signed by Dem Reps Raul Grijalva and Keith Ellison — reads as follows:

We write in strong agreement with your unwavering defense of the Democratic programs that form the bedrock of America’s middle and working classes, and which are overwhelmingly popular.

On July 7, you made very clear that “We are not going to balance the budget on the backs of America’s seniors, women and people with disabilities” and that “we do not support cuts in benefits” for vitalsafety-net programs. We agree completely.

Especially in these tough economic times, we should not be cutting Social Security, Medicare, and Medicaid benefits that millions of our constituents paid into and depend on. Such benefit cuts should be off the table in current debt discussions.

Our Republican colleagues should be embarrassed by their insistence that unless Social Security, Medicare and Medicaid benefits are cut, the nation will default on its debts. Middle-class families have sacrificed enough, and a deal that pushes the American Dream further out of reach, in order to pay for extending tax breaks for the rich and corporations, is simply unacceptable.

We are united as Democrats in saying that it’s time to stand up to the Republican hostage-taking. We will not be forced to vote for a “final agreement” that we do not agree to — and that the American people do not agree to.

We stand united with you in insisting that benefit cuts for working families, our seniors, children, and people with disabilities must be off the table, and we stand united with you in fighting for millions of Americans who need Democrats to be firmly on their side.

In another development that indicates rising opposition among Democrats, Sam Stein reports that DCCC chair Steve Israel told members at a private meeting that entitlements cuts would be damaging politically to the party, and could even hurt candidate recruitment.

To be sure, Dems have been known in the past to lay down firm markers only to let them dissolve once Obama and party leaders asked them to support a final deal that jettisoned their core priorities. But this debate is a bit different from health care. Obama and John Boehner are likely to need a sizable bloc of Dems to get the final deficit compromise through the House. Obama himself has said Democrats and Republicans must back the compromise in significant numbers. And Pelosi and other Dem party leaders also seem united against benefits cuts.

So it’s not inconceivable that a unified Dem front might be able to exert at least some leverage and have some kind of impact on the final deal. The more Dems who sign the letter, the more likely this will become — they’ll have to hurry, because we may have a deal as soon as Sunday evening — so it’ll be interesting to see what kind of numbers organizers can corral behind this effort.