Advocates of the public plan have compromised many times already, settling for the relatively weak plan that’s in the bill now on the Senate floor. And that measure seems to have more the support of more than fifty senators. In a world that were just--or even a world where the basic rules of democracy applied--that’d be the end of the story. Health care reform would pass, with that weakened but still recognizable public option still intact.

Alas, we don’t live in such a world. Instead, we live in a world where, thanks to the filibuster, it takes sixty votes to pass a bill. And thanks to the stubborn opposition of a half-dozen senators, the current proposal doesn’t have sixty votes. As a result, efforts to refashion the public option compromise continue, as they did over the weekend.

But that raises a key question: If liberals give yet more ground on the public option, what should they ask in return? So far, according to multiple Capitol Hill sources, liberals have been focusing primarily on the design of the public plan itself--that is, finding ways to make a further weakened version something that at least help fulfill the promise of the original idea.

Until very late last week, the most likely compromise along these lines was some form of the “trigger” proposal Republican Olympia Snowe first unveiled several months ago. Although the very word “trigger” has become nearly toxic, the idea actually had some merit. If constructed properly--a big if, yes--it could both impose more cost discipline on the health care system while building the foundation for a larger public option sometime down the road.

On Friday, though, Senate negotiators started talking about a new option. I wish I could tell you exactly what it is. But after talking about it with sources all weekend, I’m still not 100 percent sure--either because the details just aren’t fleshed out, the negotiators each have different notions of what the idea means, or I’m just not very good at processing what people are telling me.

What I do know is that it involves using the Office of Personnel Management to oversee some network of nationally based, non-profit plans. OPM is the department that runs the health insurance program for federal employees; as such, it has some experience (and expertise) when it comes to negotiating with insurance carriers.

How exactly the OPM’s set of plans would interface with the insurance exchanges reform would create--and why it would supposedly capture even some of the advantages of a public plan, remain a mystery to me. Jacob Hacker and Timothy Just, two experts on the subject, are both very skeptical. It's reasonable to wonder whether even some of the weakest proposals we’ve seen so far--like Snowe’s original trigger--would be better.

A lot obviously depends on the details. Maybe the new proposal contains added features that would allow it to drive down prices, introduce new efficiencies, and provide the type of security that a real public plan (even a weakened one) might.

But, if not, liberals ought to be doing something that, as Ezra Klein has noted, they don’t seem to be doing now: Using their concessions on the public option to demand improvements elsewhere in the bill. Off the top of my head, I could think of three good ones:

Improve the subsidies and/or affordability protections. Relative to the House bill, the Senate bill provides less overall financial assistance to people buying insurance. It also guarantees less comprehensive insurance. The Senate bill settled on its numbers in order to reduce the size (i.e., the price) of the overall bill. But if it were willing to put just a little more money on the table, it could bring the subsidies and affordability closer to the House levels.

Let states go early. One of the bill’s biggest flaws, as policy and politics, is the long delay between enactment and implementation. The Senate moved its implementation date back one year--to 2014--in another bid to save money. Princeton sociologist and health policy expert Paul Starr has suggested making federal funds available to those states that are prepared to start their insurance exchanges earlier. Doing so would give people assistance--and a reason to believe in reform--a lot more quickly.

Go back to industry to get more money. It’s pretty clear that the drug industry got off easy in its deal with the Finance Committee and the White House. The Senate could revisit that arrangement--and the hospital industry deal, too. It could also ask the device industry to put more money on the table. In each of these cases, revising the existing deals could produce legislation that not only frees up money but also speeds improvements to the health care system itself.

The obstacle to doing the first two is money. Within the Senate, there’s strong opposition to putting more spending on the table--in part because of President Obama’s statement, during his September speech, that a health reform should cost around $900 billion. (That statement looks worse and worse every day.) But the statement was vague enough to allow some wiggle room--and, besides, revisiting the industry deals would yield new savings, which would free up more money.

Of course, all of this assumes liberals can extract a deal--that the Ben Nelsons, Joe Liebermans, and Olympia Snowes don’t hold all the leverage. But liberals aren’t completely powerless here. At the end of the day, even the centrists seem to grasp the wisdom of passing a bill, if only for the sake of vindicating Congress' ability to solve a major national problem.

Since the centrists going to score a big victory no matter what, they might be willing to give a little ground--if not on the public plan itself, then on one of these other issues. But we'll never find out if the liberals don't ask.

Update: Speaking of Ezra, he's got a fleshed-out explanation of the OPM option--as well as word that progressives are, in fact, starting to broaden the discussion. He's also hearing (as am I) that the idea attracting the most attention right now is allowing older workers to buy Medicare coverage. It's a very good idea that really would offer at least some workers--those over 55, if it's like past proposals--the opportunity to enroll not in a public plan but the public plan, namely Medicare. (Two key questions: Would workers be eligible for subsidies to offset the cost; if so, when?) Staff for the so-called Team of Ten, a liberal-conservative group of Senators meeting to hash out this deal, were supposed to convene this morning, with the members scheduled to meet again this afternoon. So stay tuned.