Senate Majority Leader Harry Reid and his top lieutenant Chuck Schumer joined 29 Senate Democrats in all, pledging to oppose any cuts to Social Security inside a broader deficit package. Several model deficit reduction packages, like the Bowles-Simpson recommendations, included cuts to Social Security benefits in the form of changing the calculation of the cost of living benefit to “chained CPI,” which would reduce payments, or increases in the retirement age, which works out to a net benefit cut.

The letter forms a significant marker as Congress looks toward a possible deficit bargain in the lame-duck session after the election. It says Social Security has problems down the road, but that they should be dealt with separately from any budget deal.

Cuts to Social Security and other entitlements are seen as key to getting the bipartisan cooperation of Republicans in any deal, just as revenue increases are key for Democrats.

The problem here is that only 29 members signed this, rather than the 41 that would be needed to block through a filibuster any deficit reduction package. If this gets through the lame duck session, you may have a higher percentage of Democrats willing to sign on to such a pledge with the incoming freshman Senators. But a substantial number of Democrats withheld their signature from this pledge, which was organized by Sheldon Whitehouse (D-RI), Bernie Sanders (I-VT), Mark Begich (D-AK) and Al Franken (D-MN).

Here are the others who signed:

Sens. Jack Reed (D-R.I.), Sherrod Brown (D-Ohio), Ron Wyden (D-Ore.), Patrick Leahy (D-Vt.), Ben Cardin (D-Md.), Debbie Stabenow (D-Mich.), Richard Blumenthal (D-Conn.), Tom Harkin (D-Iowa), Jeff Merkley (D-Ore.), Frank Lautenberg (D-N.J.), Barbara Mikulski (D-Md.), Patty Murray (D-Wash.), Barbara Boxer (D-Calif.), Maria Cantwell (D-Wash.), Daniel Akaka (D-Hawaii), Tim Johnson (D-S.D.), John D. Rockefeller IV (D-W.Va.), Daniel Inouye (D-Hawaii), Tom Udall (D-N.M.), Robert Menendez (D-N.J.), Carl Levin (D-Mich.), Kirsten Gillibrand (D-N.Y.) and Joe Manchin III (D-W.Va.) also signed on.

Notice the presence of Manchin there. So who didn’t sign?

Max Baucus, Michael Bennet, Jeff Bingaman, Tom Carper, Bob Casey, Kent Conrad, Chris Coons, Dick Durbin, Dianne Feinstein, Kay Hagan, John Kerry, Amy Klobuchar, Herb Kohl, Mary Landrieu, Joe Lieberman, Claire McCaskill, Ben Nelson, Bill Nelson, Mark Pryor, Jeanne Shaheen, Jon Tester, Mark Udall, Mark Warner, Jim Webb.

Bingaman, Conrad, Ben Nelson, Kohl, Lieberman and Webb are retiring, and assuming they get replaced by Democrats (except for Nelson, because that’s not happening), I think we can expect Tammy Baldwin (WI), Martin Heinrich (NM) and Chris Murphy (CT), at least, to stand with the “no cuts” caucus. Tim Kaine not so much, and frankly I don’t know enough about North Dakota candidate Heidi Heitkamp to make a determination. There’s a “fixing the fiscal mess” section of her website so that’s not terribly encouraging.

Elizabeth Warren is probably in the “no cuts” caucus, so that’s a pickup. Shelley Berkley may or may not be, if she wins in Nevada. So the best case here is for five more votes out of the elections, if they break the right way. Tester, McCaskill, Bill Nelson and Casey, if they get through their elections, may change their tune a bit, but who knows.

So the best case, putting let’s say 2 of the Tester/McCaskill/Bill Nelson/Casey axis into the “no cuts” camp, is still 36 votes, five shy of what would be needed. However, the inclusion of Reid on this list is huge, because as Majority Leader he would be in a position to block any votes on such a deal, if he so chose.

I think the real tell here is this: the two Senators arguably the closest to the White House, Dick Durbin and John Kerry, did not sign this letter. That’s a bit of a tell for the level of flexibility the President wants on a grand bargain. The simple claim made in this letter is that the budget and Social Security have separate funding streams, and so changes to their programs should be considered separately. But the two biggest allies of the White House in the Senate wouldn’t even go there.