Toward the end of the summer, when it was unclear whether Senate Majority Leader Harry Reid would include a public option in his health care bill, progressives let it be known that he would not be forgiven if he allowed a handful of nameless Democrats silently filibuster the provision. In the end, this pressure, and various other factors, ultimately convinced Reid to include the opt out public option in the legislation, and the opponents have had to come forward. Their names won't surprise students of Democratic politics: Sens. Joe Lieberman (I-CT), Blanche Lincoln (D-AR), Ben Nelson (D-NE), and Mary Landrieu (D-LA).

These conservative Democrats are known for taking stances at odds with the party on key issues, but in this health care debate they are ultimately driven by very different motives. They have suddenly become the targets of every major reform organization in the country, and understanding what makes them tick will be key to the advocates who are now trying to change their minds.

Joe Lieberman

Lieberman may be the trickiest of the four to secure. The moment Reid announced he'd included a public option in the Senate bill, Lieberman charged out of the gate to announce that he'd filibuster a health care bill with any kind of government plan in it: opt-out, opt-in, triggers--you name it. Immediately, speculation began to fly regarding what, exactly, had animated Lieberman, who after all represents a blue state whose voters support the idea. Some believe his gambit is rooted in his 2006 split with the party, and his 2008 decision to campaign for the McCain-Palin ticket. Others believe he's gotten too cozy with the insurance industry, which still has a heavy presence on Connecticut. Unexplored is the possibility that he--already a black sheep in Democratic politics--was simply giving his centrist friends cover. Those centrists--profiled below--would like a Republican (a.k.a. Olympia Snowe) to vote for this bill, too, and the only way to assure that her preferences receive maximum attention is to signal, clearly, that at least one Democrat isn't on board with the plan. Whether orchestrated or not, that person is Lieberman. He's been the most adamant against the public option of any of his peers.

Blanche Lincoln

Lincoln was cautiously supportive of the public option throughout most of the summer. In fact, on the day she announced her intent to filibuster a"government-run" insurance option, her website, embarrassingly, still boasted of her support for the very measure she was threatening to obstruct. What explains this curious mixed message? Unlike Lieberman, Nelson, or Landrieu, Lincoln is facing a tough re-election right now. She's going to be attacked for supporting a "government takeover" of health care no matter what, and would like to present her conservative constituents with a scalp to prove she didn't roll over for the liberals in her party. It's an immediate political calculation. Getting her on board will require convincing her she stands more to lose by blocking the provision than by allowing a vote on it.

Ben Nelson

Nelson, as I've noted before, is simply the most conservative Democrat in the caucus. He wants the bills he votes for to have Republican supporter(s), and he always prefers the option that liberals in the party don't: less stimulus over more stimulus, triggers over the public option, opt-in over opt-out. Nelson held out for a long time before agreeing to debate the bill this past weekend. It's conceivable that the prospects of failure down the line will make the pressure on him and other conservative Democrats so great that he'll agree not to filibuster. But, again, if Lieberman sticks to his guns, the compromise might just happen anyhow.

Mary Landrieu

Landrieu has been extremely candid about her reluctance to support a public option. Her constituency is very broad, she's mindful of the warnings of industry, she has leverage, and she's using it. But...she was just re-elected. It's almost inconceivable that her vote on health care in late 2009/early 2010 will matter very much when she's up for re-election in 2014. Strictly on political terms, she should be a company Democrat right now, though it's unclear if the political consideration is all that's driving her decision-making on the issue.

These are the four Democrats threatening to filibuster a public option bill down the line. They're also in discussions with leadership and Sen. Tom Carper (D-DE) regarding a compromise modeled on Snowe's trigger. How they change the bill so significantly remains unclear (can Reid round up 60 votes to swap the provisions? Does he pull the bill off the floor and reintroduce it with a different public option?) For the time being, though, liberals are turning up the heat on these four. And to succeed, they'll need to be well aware of what buttons to push.