WASHINGTON — For now, Sen. Joe Lieberman, the Democrats' 2000 vice presidential nominee, is still welcome at Senate Democrats' weekly lunches -- even if he is actively campaigning for Republican presidential candidate John McCain.
But the welcome mat may not be out for long.
If Democrats expand their Senate majority in November, the Connecticut senator could find himself in a political no man's land. But at least until then, he holds a coveted committee chairmanship and has attracted no hint of retribution. After all, Lieberman, an independent who caucuses with the Democrats, is crucial to their maintaining their tenuous 51-49 Senate majority.
"If it weren't for Joe Lieberman, we'd be in the minority," said Sen. Tom Harkin (D-Iowa).
Lieberman acknowledged in an interview that his support for McCain has created friction with Democrats.
"Sure, it's somewhat awkward," he said. "What I did was unconventional. I understand that. I'm a Democrat -- an independent Democrat, but still a registered Democrat -- supporting a Republican."
In fact, he's become a key Republican Party asset.
Lieberman was in that role Wednesday, defending McCain against Democratic charges that he undervalued the importance of bringing U.S. troops home. And Lieberman took a swipe at Democratic presidential candidate Barack Obama for being "consistently wrong" on Iraq.
Lieberman is heading up a new group to woo Democrats and independents to McCain. He's even offered to speak at the GOP convention.
"It hurts," Sen. Dianne Feinstein (D-Calif.) said of Lieberman's support of McCain. Sen. Ron Wyden (D-Ore.) added, "Clearly, we wish he was with Sen. Obama."
Obama last week took Lieberman to a corner of the Senate chamber to talk on the same day that Lieberman joined Republicans in criticizing the presumptive Democratic presidential nominee's foreign policy positions.
In the interview, Lieberman said he was backing McCain because they agree "almost totally" on national security issues such as the war in Iraq, and he sees the Arizona senator as more able to work across party lines to get things done.
But Lieberman also said the Democratic Party had changed since he was Al Gore's running mate in 2000, becoming more partisan and left-leaning. Still, he said he had no interest in becoming a Republican.
Lieberman won reelection to the Senate in 2006 as an independent after losing the Connecticut Democratic primary because of his support for the war. A number of his Senate Democratic colleagues abandoned him to support the Democratic nominee, Ned Lamont.
That election, in some measure, "liberated me to be totally independent," Lieberman said.
He knows his actions have upset many Democrats.
"A lot of them are not happy, I understand that," he said.
The Connecticut Democratic Party has already stopped inviting Lieberman to its events. The state's Democratic Party chairwoman, Nancy DiNardo, said many members were upset about Lieberman's support for McCain. But she has no desire for retribution.
"If we start going after our people because they aren't agreeing with us on various issues, then we become the Republican Party," she said.
Senate Democratic Leader Harry Reid of Nevada said last week that he had talked with Lieberman about his stance.
"But I'm not about to threaten anybody at this stage," he added.
Lieberman said that on most issues, he's been a good ally of Senate Democrats. Democratic leaders tend to agree.
"Joe Lieberman is a dutiful member of our caucus and provides critical votes on a regular basis," said Sen. Richard J. Durbin of Illinois, the Senate's No. 2 Democrat, noting that the main sources of disagreement are Lieberman's support for the war and McCain.
Still, Democratic disappointment is obvious.
"At some level, there is some understanding," said Sen. Claire McCaskill (D-Mo.), apparently a reference to Lieberman's longtime friendship with McCain and Lieberman's and McCain's support for the war. "At another level, it's awkward. And on another level, it's frustrating."
Lieberman said he initially planned to delay any presidential endorsement until the parties chose their nominees. But McCain called him after Thanksgiving and asked for his help.
Lieberman said his wife, Hadassah, kidded him at the time about getting himself into hot water.
"She said at least this won't last long, because McCain's obviously not doing well," he said.
Republicans relish Lieberman's support of McCain. As the first Jewish vice presidential nominee on a major party ticket, Lieberman could help McCain among Jewish voters, who could be pivotal in some swing states, including Florida.
In a political body like the Senate, fraternal tensions are par for the course. Although Lieberman's actions are unusual, Zell Miller, a conservative Democratic senator from Georgia, infuriated his party by endorsing President Bush for reelection and delivering a fiery keynote speech at the 2004 Republican convention.
Miller declined to comment for this article.
On the GOP side, former Sen. Lincoln Chafee, a moderate Republican from Rhode Island, irritated GOP leaders by breaking with the party on a number of issues, then voting in 2004 against Bush.
But two years later, the Senate GOP campaign committee tried -- unsuccessfully -- to help him win reelection in a year of Democratic gains.
Chafee suggested Lieberman's actions might be testing the Democrats' patience.
"How far can you take this independence and still break bread with the Democrats?" he asked.
Despite the history, retaliation has been rare.
"Senators have been reluctant to punish colleagues for straying from the party line," said John J. Pitney Jr., a professor of government at Claremont McKenna College.
On the other hand, Pitney said, "suppose Obama loses narrowly while Senate Democrats gain seats. At that point, Lieberman's critics will have both the motive and the means to punish him."