The reason this is a little scary for Democrats is because the usual things that serve to motivate a Congressman don't seem to motivate Joe Lieberman.
Would voting to filibuster the Democrats' health care bill (if it contains a decent public option) endear Lieberman to his constituents? No; Connecticutians favor the public option 64-31.
Would it make his path to re-election easier? No, because it would virtually assure that Lieberman faces a vigorous and well-funded challenge from a credible, capital-D Democrat, and polls show him losing such a match-up badly.
Would it buy him more power in the Senate? No, because Democrats would have every reason to strip him of his chairmanship of the Homeland Security Committee.
Is Lieberman's stance intended to placate the special interests in his state? Perhaps this is part of it -- there are a lot of insurance companies in Connecticut -- but Lieberman is generally not one of the more sold-out Senators, ranking 75th out of the 100-member chamber in the percentage of his fundraising that comes from corporate PACs.
Are there any particular compromises or concessions he wants in the bill? He hasn't stipulated any, at least not publicly.
Might he have a legitimate policy objection to the public option? Certainly there are some legitimate objections -- whether or not you agree with them. But Lieberman's objections don't make any sense. He says he's worried about blunting "the economic recovery we’re in" even though the public option provisions wouldn't kick in until 2013. He says he's worried about debt-reduction when the public option would make the bill more deficit-neutral. And he campaigned on a public-option type alternative called "MediChoice" in 2006.
What Joe Lieberman wants, in all probability, is attention. He wants Harry Reid to have to stand up and say things like : "I don't have anyone that I've worked harder with, have more respect for, in the Senate than Joe Lieberman." He wants face time on Meet the Press. He wants to make liberals feel some pain -- especially those who tried to get Ned Lamont elected in his place. He wants everyone to know how maverick-y he is.
But even if Lieberman will probably cave, this creates real problems for Democrats. For one thing, Lieberman has said he won't oppose the motion to proceed with the health care bill. Instead, he'll filibuster the end result, if he doesn't like the outcome. This is actually very devious. If Harry Reid determines that he doesn't have 60 votes on the motion to proceed on a bill with a public option, then he doesn't have to bring it to the floor. Progressives will be apoplectic, Reid will lose some face, pundits will talk about Democrats having lost momentum -- but the prospects for health care reform will probably not have seriously been damaged. The opt-out will get replaced by an opt-in or maybe a trigger or co-ops or whatever and Olympia Snowe will be very happy and the debate will proceed.
But if, on the other hand, Lieberman filibusters the vote for final passage, that will have come after weeks of floor debate, amendments, and compromising on all sorts of issues. This would be a very, very serious blow to health care reform. And it makes this a much more expensive bluff to call.
The other way that this is damaging to Democrats, of course, is that it may embolden an Evan Bayh or a Blanche Lincoln or a Ben Nelson to adopt Lieberman's stance. None of these guys want to be the lone Democratic member to filibuster -- but it's much easier to defray individual responsibility on a procedural vote against your party when you have someone else joining you.
But while a Nelson or a Lincoln is liable to have a fairly rational set of concerns -- basically, they want to ensure they get re-elected -- it's tough to bargain with people like Lieberman who are a little crazy. In certain ways, he resembles nothing so much as one of those rogue, third-bit Middle Eastern dictators that he's so often carping about, capable of creating great anxiety with relatively little expenditure of resources, and taking equal pleasure in watching his friends and enemies sweat.