TNR contributor Michael Idov has a terrific profile of Joe Lieberman in New York magazine. Here's Lieberman explaining why he's backing John McCain: 

The real reason he’s backing McCain, Lieberman says, is because he believes in the kind of foreign policy that the Democrats don’t provide anymore: unflinching on Iraq, Iran, and Russia, and unfailingly loyal to Israel (he invokes Nixon’s line about “loading every plane” with weapons for Israel to explain what kind of president McCain will be). Lieberman believes foreign policy is the defining issue of the day, and sees Obama’s nomination as the regrettable result of a knee-jerk, blog-fueled peacenik mentality among the Democrats. 

This makes sense on its own terms. (Though even an Israel hawk like me finds the monomaniacal focus on what's best for Israel a bit disturbing.) But, of course, during his 2004 and 2006 campaigns, Lieberman insisted that liberals should not judge him solely on "one issue," and must instead take the whole policy landscape into account. He was quite sanctimonious on this point. Now he's doing exactly what he was so indignant about others doing to him.

Idov has another passage that, I think, perfectly captures Lieberman's mentality:

“In 2007,” he tells me, “netroots and MoveOn.org controlled the agenda—they endorsed Obama like they endorsed Ned Lamont, and did to Hillary what they did to me in 2006.” Lieberman, who often brings up Lamont without provocation, seems to view the McCain-Obama matchup as his battle with Lamont writ large on the national canvas: a voice-of-reason maverick beholden to no one but his conscience pitted against a cocky line-cutter with no experience. “The lesson Joe learned about the netroots,” says a onetime colleague, “is now the frame he will put around any situation, even when it doesn’t apply.”

Update: Jason points out to me that Lieberman endorsed McCain in 2007, when Hillary Clinton was the front-runner, so there's obvious revisionism in his implication that he turned to McCain because radical netroots tool Barack Obama took over the Democratic party.

--Jonathan Chait