By Richard Allen Greene
BBC News, Washington
Washington insiders from both parties have long been muttering that George W Bush's Republican party is going to lose ground in elections in November.
The loss was a humiliation for the long-standing senator
The only question, they say, is how much ground. Enough to lose control of one or both houses of Congress?
They may have had the beginnings of an answer on Tuesday night, but the scalp that was claimed was not Republican - it was Democrat Joe Lieberman.
His support for the Iraq war was a major factor in his loss, experts say.
Mr Lieberman's opponent, political novice Ned Lamont - who was unknown six months ago - campaigned hard on an anti-war platform and became the first man in decades to beat an incumbent senator in a primary race.
Although Mr Lamont's margin of victory was narrow, this was a shock loss for the Connecticut centrist who ran for vice-president only six years ago.
Mr Lieberman has said he will run as an independent in November.
'Bush's favourite Democrat'
A long-time Lieberman ally turned Lamont backer said it was Mr Lieberman's support for the Iraq war and the Bush administration that led to his defeat on Tuesday.
"Connecticut opposes this war and continues to oppose this war," said Irving Stolberg, who has been active in state Democratic politics for more than 35 years.
Smiles then, but Mr Clinton could not save Mr Lieberman
"You can't run in Connecticut as Bush and Cheney's favourite Democrat."
Analysis by the New York Times newspaper showed that Mr Lieberman has voted with the Democrats in the Senate the overwhelming majority of the time during his career.
Connecticut's newspapers endorsed him, as did unions and many other traditionally Democratic groups.
Bill Clinton led the list of party luminaries who visited the state to campaign for him.
Kiss of death?
But Mr Lamont's campaign relentlessly linked him to the Iraq war - of which he has been an unwavering supporter - and to President Bush.
Lamont television ads showed Mr Bush appearing to kiss Mr Lieberman on the cheek after his State of the Union address last year.
The race brought unknown Ned Lamont into the limelight
Tom Matzzie of the liberal activist group MoveOn.org - which endorsed Mr Lamont - said the result reflected voter perception that the Democratic senator was too close to the Republican president.
"If you are a rubber stamp for George Bush, then you are going to be in trouble on election day," he said.
Voters "are saying they want leaders who are going to stand up and fight against the Bush administration, especially the war in Iraq", he added.
National party mood
Connecticut is among the most reliably Democratic states in the US, political scientist Larry Sabato said, pointing out that Mr Bush lost the state by a landslide in both 2000 and 2004.
Mr Lieberman's defeat reflects how the Democratic party feels, he said - and even before the votes were tallied, centrist Democrats who shared Mr Lieberman's positions were tacking to the left to take the party's mood into account.
1942: Born in Stamford, Connecticut
1970: Elected to Connecticut State Senate
1983: Elected as Connecticut attorney general
1989: Wins bid for US Senate
2000: First Jewish candidate for vice president
Unsuccessful bid for 2004 presidential nomination
"Hillary Clinton has already gotten the message from the Lieberman-Lamont race," he said, noting that last week she called for the resignation of Donald Rumsfeld, Mr Bush's defence secretary.
Top Republican strategists may take short-term delight in Mr Lieberman's humiliation, hoping it will enable them to continue portraying the Democrats as weak on defence.
But Mr Sabato said it was the Republicans who had the most to fear from the unpopularity of the president and the war.
"The real question is what happens with Bush and Iraq through November.
"If the war somehow ceases to be the disaster that it is, and if Bush somehow manages to rise to something close to 50% [approval rating], then this could backfire on the Democrats.
"But as long as Bush and the war remain unpopular, then this will not be a problem for the Democrats in 2006," he predicted.
Mr Sabato cautioned against reading too much into the results of one election in one small liberal north-eastern state.
"Americans are going to be interested in this result, but this is not determinative of how people will vote outside of Connecticut."
But Mr Matzzie of MoveOn.org said any politician too close to Mr Bush would suffer the same fate as Mr Lieberman.
"There are millions of disgruntled Republicans and independents around the country. Why didn't this happen to other politicians around the country who shared Lieberman's position on the war?
"Because they were more critical of the president."