The four-term Connecticut senator, who came tantalizingly close to being Al Gore's vice president in 2000, not only has been campaigning for his pal, presumed Republican nominee John McCain, now he's publicly criticizing the Democrats' standard-bearer, Barack Obama. Lieberman has strayed before, most notably switching from Democrat to independent in 2006 to hold onto his Senate seat after a Democratic primary loss.
But the latest betrayal has upset Democrats, who often answer in clipped but polite tones when asked about Lieberman. The reason:The independent still caucuses with the Democrats on most issues except the Iraq war, and he holds their slim political majority in his hands.
"There's a commonly held hope that he's not going to be transformed into an attack dog for Republicans," said Sen. John Kerry, D-Mass., an Obama supporter.
Lieberman has wasted no time in questioning Obama's positions on Iran and Israel, two topics on which Lieberman and McCain agree.
Just one day after Obama clinched his party's nomination, Lieberman joined Republicans on a McCain campaign teleconference call assailing Obama following his foreign policy address to a leading Jewish group.
Lieberman accused Obama of blaming U.S. policies for "essentially sort of strengthening" Iran.
"If Israel is in danger today, it's not because of American foreign policy, which has been strongly supportive of Israel in every way," he said. "It is not because of what we have done in Iraq. It is because Iran is a fanatical terrorist, expansionist state."
Later that day, during a budget vote in the Senate, Obama led Lieberman to a corner of the Senate floor for a pointed private conversation. Without elaborating, Obama told reporters the chat was about politics. Senate Majority Leader Harry Reid, D-Nev., had a similar private conversation with Lieberman.
For his part, Lieberman said he assured Obama he would avoid personal attacks.
"I said, and we agreed, that any time I get out there mostly I'm going to be talking positively about John McCain — and anytime I would take issue with Barack Obama, it would never be personal because I have the highest regard for him personally," he said.
Still, Democrats were irked. Lieberman seemed to be breaking new ground — shifting gears from simply promoting McCain to taking shots at Obama.
"I'm glad that Barack Obama had a direct conversation with Joe," Sen. Dick Durbin, Obama's fellow Illinois senator, told reporters. "I hope that Joe will realize that even though he's a friend of John McCain's and feels differently on the war, there are so many other issues Barack stands for that have been a part of Joe's career."
Lieberman's Connecticut colleague, Democratic Sen. Chris Dodd, said he's heard McCain talk about keeping a civil tone to the campaign.
"It might be a good message for him to convey to his supporters," said Dodd, also an Obama supporter.
Obama had backed Lieberman in the 2006 Democratic Senate primary in Connecticut. After he lost to Ned Lamont, an anti-war candidate, Lieberman defied party leaders and ran as an independent in the general election. Leading Democrats — Obama, Dodd and Kerry among them — then backed Lamont. Lieberman was re-elected with support from the GOP, including praise from the White House and fundraising help from prominent Republicans.
Oddly, Lieberman befriended and dispensed advice to Obama when the Illinois senator arrived in Washington in 2005.
"We have established a very good relationship," Lieberman says. "I have a lot of affection for him."
Call it Lieberman's version of tough love.
The Connecticut lawmaker is willing to speak at the Republican convention this summer if McCain asks. He also has been mentioned as a potential McCain running mate.
Democrats have reason to tolerate Lieberman's actions. If he were to caucus with the GOP, the balance of power in the narrowly divided Senate would slip away, especially with Sen. Edward M. Kennedy, D-Mass., battling brain cancer. Democrats need Lieberman to maintain their 51-49 Senate majority.
Beyond Iraq, Lieberman tends to vote with Democrats on major issues.
"Joe and I have known each other 40 years," said Dodd. "On almost every issue, Joe is a mainstream Democrat."
There is speculation that if Democrats bolster their Senate majority this fall, they could seek payback by stripping Lieberman of his Homeland Security and Governmental Affairs Committee
While there's no serious talk afoot about punishing Lieberman, Kerry said, "I can't tell you what happens next year."