Remarking on the news of Lieberman’s retirement, Jonathan Chait wonders why Lieberman endorsed McCain in 2008:

The most interesting question may be why Lieberman took this suicidal path. My guess would be that he didn’t consider it suicide. Lieberman is a true believing New Democrat who is influenced by the neoconservatives. One common thread uniting these two strands of thought is an overly-developed fear of McGovernism. George McGovern, the very liberal Democratic nominee in 1972, lost in a landslide, and his defeat ever since has been held up as evidence that middle America rejects and always will reject unvarnished liberalism. I think there’s some truth to that but it’s an oversimplified view.

The point, though, is that Lieberman is almost certainly a true believer in the legend. And you have to remember that, when Barack Obama won the Democratic nomination, a lot of centrists and neoconservatives viewed him as the heir to McGovern and a likely loser. In Lieberman’s mind, I would submit, Obama was the heir to McGovern, and after he went down to defeat at the hands of popular maverick John McCain, Lieberman would be well-positioned to say “I told you so.” He could then tell Democrats that only his brand of moderate Democratic politics could truly prevail, and the sadder but wiser party base would trudge back to his column.

Chait’s speculation is plausible, but I would add a little more. Lieberman’s decision to back McCain later on was very likely influenced by the debate over the “surge” in early 2007. Along with Lieberman, McCain was one of the most vocal supporters of the new plan, and Obama was one of many to voice skepticism and opposition to it. By the summer of 2008, a fairly conventional, mistaken view on the right and in many mainstream media organizations was that the “surge” had “worked,” and that Obama had judged incorrectly. This gave Lieberman something specific to which he could link his (baseless) fear of a new McGovern.

When Lieberman returned to the Senate in 2007, he had just come out of a general election in which practically the only people in the national media who vocally supported him were McCain’s reliably hawkish supporters in the GOP, including many prominent neoconservatives. These had been the people wailing and gnashing their teeth about the “purge” of Lieberman in the summer of 2006, and they became Lieberman’s cheering section because of their common support for the war in Iraq. Lieberman’s victory was their one consolation in an election that drove the GOP out of power largely because of the war in Iraq. The ideological and political alliance between Lieberman and hawkish interventionists in the GOP became stronger than any residual partisan attachment Lieberman might have had, and so endorsing McCain must have seemed the obvious sequel to his independent run for the Senate. For that reason, my guess is that Lieberman would have backed McCain even if Clinton had been the Democratic nominee, because she had also opposed the “surge,” which was an inexcusable error for people who viewed the war as Lieberman did.