UNDERSTANDING LIEBERMAN’S MOTIVATIONS…. Whenever Joe Lieberman’s name pops up, the now-tired cliche — he’s with Democrats on “everything but foreign policy” — still pops up. Usually, it’s intended as sarcasm, emphasizing the fact that Lieberman is at odds with his former party on a variety of issues that have nothing to do with his neo-conservative worldview on international affairs.

But the reason the cliche came into existence in the first place is that, on domestic policy, Lieberman has actually maintained some pretty progressive ideas. On issues like gay rights and the environment, Lieberman has occasionally even been downright liberal.

When it comes to health care policy, the Connecticut senator wasn’t nearly as conservative as he is now. So what happened? Peter Beinart’s explanation sounds pretty compelling.

For close to a decade, he got nearly perfect scores from the American Public Health Association, which backs a single-payer health-care system, and in lieu of that, the “public option.” Now, all of a sudden, he’s so outraged by a public option that he’s threatening to filibuster any bill that contains it. […]

So why is he doing this? Because he’s bitter. According to former staffers and associates, he was upset by his dismal showing in the 2004 Democratic presidential primary. And he was enraged by the tepid support he got from many party leaders in 2006, when he lost the Democratic primary to an anti-war activist and won reelection as an independent. Gradually, this personal alienation has eaten away at his liberal domestic views. His staff has grown markedly more conservative in recent years, and his closest friends in Congress are now Republicans John McCain and Lindsey Graham. For Lieberman, the personal has become political, and it has pushed him further to the right.

The irony is that when Lieberman was officially a Democrat, he was ideologically independent — a living manifestation of the Humphrey-Jackson tradition. Now that he’s technically an independent, he’s becoming a standard-issue conservative. For people who believe — as Lieberman himself once did — in progressive health-care reform, it’s a tragic shift.

This would explain a few things. For example, Lieberman has been making arguments that don’t make any sense, as if he’s confused about the policy basics on an issue he’s studied for years. Why would that happen? Because, by this reasoning, he’s letting his emotions override his judgment — Democrats hurt his feelings, so he’ll teach them a lesson. It’s small and petty, and would undermine the interests of millions of struggling Americans, but in a contest between Lieberman’s ego and the public benefits associated with increased competition in the health care marketplace, it’s apparently not even close.

Of course, Beinart may be mistaken. Tim Fernholz noted that Lieberman may simply be “in hock to insurance companies,” some of which are located in Connecticut.

That isn’t much of a choice, I’m afraid. Lieberman is either putting his hurt feelings over the needs of the nation, or he’s putting the insurance industry’s profits over the needs of the nation. Either way, Lieberman is so far from the man he used to be, the two bear no resemblance.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.