LIEBERMAN’S NOT LEAVING; HE’S PUSHING THE JOEMENTUM IN ANOTHER DIRECTION…. More than a few folks on the left were relishing the opportunity to defeat Sen. Joe Lieberman (I) in Connecticut next year. It appears they won’t get the chance — the senator will announce today that he will not seek a fifth term.

Senator Joseph I. Lieberman of Connecticut, the Democratic vice-presidential candidate in 2000 who later became deeply alienated from his party, will announce on Wednesday that he will not seek a fifth term, according to people he told of the decision.

Mr. Lieberman, 68, whose term is up in January 2013, has chosen to retire rather than face a difficult campaign for re-election, according to aides and others who spoke to the senator on Tuesday.

A source close to Lieberman told the New York Times that the senator decided retirement was preferable to defeat. “I don’t think he wanted to go out feet first,” the person said.

It’s an important point. By all accounts, Lieberman’s re-election prospects were very poor — he won in 2006, after losing the Democratic primary, because it was effectively a two-way contest between Lieberman and Ned Lamont. Lieberman had enough GOP and independent support, along with some lingering Democratic backing, to win with relative ease. That wasn’t going to happen in 2012 — both major parties intend to run top-tier candidates — and Lieberman’s standing has weakened considerably in recent years, with moves that managed to annoy practically everyone.

In terms of a legacy, Lieberman will depart as a prominent national figure, with a level of public notoriety most senators never achieve. But in liberal/Democratic/progressive circles, the senator will very likely be remembered as a source of near-constant frustration and disappointment.

Regular readers know that I’ve long found Lieberman to be hopelessly exasperating, but I’m willing to concede his record has some high points. Just last month, Lieberman showed great leadership on DADT repeal, and was instrumental in getting it passed. Lieberman also had a very strong record on reproductive rights, which is more than can be said about many of his “centrist” brethren, and played a constructive role in helping defeat the GOP filibuster of the 2009 Recovery Act.

But then there’s the rest of Lieberman’s record. The cliche, repeated by many Democratic leaders over the years, is that Lieberman is “with us on everything but foreign policy.” To be sure, when it came to national security, wars, and international affairs, the Connecticut Independent was an even more reliable Republican vote than some Republicans. There was literally no difference between Lieberman’s vision and that of the Bush/Cheney team, and his allegiance with conservative Republicans like John McCain and Lindsey Graham on foreign policy was as misguided as it was maddening.

But the cliche was also wrong. For years, even when he was an actual Democrat, Lieberman adopted positions well to the right of his party on school vouchers, “tort reform,” the Bush White House’s “faith-based” initiative, gun control, and the “blame Hollywood” effort. Lieberman flirted with conservative changes to Social Security. He was the first Democrat to go after then-President Clinton during the Lewinsky scandal. During the uproar over Terri Schiavo, Lieberman was even sympathetic to Republican efforts.

Perhaps most strikingly, Lieberman was chiefly responsible for killing the public option in the health care debate, insisting at the outset that he would kill the entire initiative over this one, popular idea, and coming up with a series of bizarre rationales for his position.

Looking ahead, Democrats are optimistic about winning Lieberman’s seat next year, and with good reason. The result will very likely be a reliable progressive voice, and a more consistent, less sanctimonious, lawmaker.

Postscript: I almost forgot that Lieberman was responsible for the most unintentionally hilarious bit of political spin I’ve ever heard. In 2004, as part of an inexplicable Democratic presidential campaign, Lieberman was counting on a strong showing in New Hampshire. He came in fifth, with just 9% support. Heralding the results to supporters the night of the primary, Lieberman proudly proclaimed, “We are in a three-way split decision … for third place.”

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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.