Leading article: A defeat that transforms the electoral landscape

The defeat of Joe Lieberman in his bid to be re-elected Senator for the state of Connecticut marks a turning point in US politics of the post-9/11 era. Senator Lieberman, a respected figure nationally, fell at the first hurdle, succumbing to the challenge mounted by a novice politician, Ned Lamont, for the Democratic nomination. If Mr Lieberman's name appears on the ballot paper in November, it will be as an independent, without the endorsement of the party he has represented since he entered politics more than 35 years ago.

Mr Lieberman's defeat, however, is at once both more and less significant than the bare fact of Mr Lamont's successful challenge would suggest. It is of infinitely more significance because of Mr Lieberman's stature and because of the issue on which he fell. Until recently one of the most respected politicians in the United States, Mr Lieberman had a reputation for personal integrity and was known for his expertise in security and defence. His appeal crossed party lines. This, and his political solidity, were the reasons why he was selected as Al Gore's running-mate for the presidency in 2000. Joe Lieberman is not just any rank-and-file US politician; on his home turf, he should have been unbeatable.

What cost Mr Lieberman the Senate nomination, however, was his vocal support for the war in Iraq. Mr Lieberman became the war's Democratic cheerleader to the point where he was seriously mooted as a replacement for the Defence Secretary, Donald Rumsfeld, should he be removed or resign. As the violence in Iraq escalated and more and more Democrats voiced criticism, Mr Lieberman steadfastly kept the faith.

Mr Lamont based his challenge on the single issue of Iraq. It was, he argued, a grave mistake for the Democratic Party, and for their state Senator, to have supported the war at all. Tuesday's primary became in effect a referendum on Iraq. The result showed that opposition to the war among Democrats of Connecticut cancelled out all Mr Lieberman's other political assets. It also signalled to all election candidates across the United States - but especially to Democrats - how far the national mood had shifted. From now on, support for the war must be accounted a political liability.

Democrats in Congress have tried in recent months to adopt a more critical public stance. In so doing, they have faced a problem similar to the one that faces the Conservatives here: how to justify their initial support for the war in the light of what has happened since. They charge incompetence and hubris, applied to the aftermath - but not the principle - of the war. But it is not an easy switch to make; there could be other political casualties in the US before the season is out.

Mr Lieberman's defeat is also less significant than it might appear because evidence from Connecticut that the Iraq war will be an issue - if not the issue - in the mid-term elections will not necessarily help the Democrats in their efforts to win back the two houses of Congress in November. Yes, his defeat testifies to a sharp change in US public sentiment, and it stands as a warning to other candidates - Republicans as well as Democrats. It may even have negative implications for Hillary Clinton if she decides to run for President in 2008. But it also presages divisions in the Democratic Party over Iraq policy that could only weaken it as the mid-term campaign progresses. The prospect that the Democrats will adopt an election platform which unanimously opposes the war is tantalising, but ultimately remote. Only when the Democratic Party summons up the courage to contest Mr Bush's abuse of the "patriot" card will the shadow of 9/11 be lifted from US politics.