Ned Lamont has floppy hair, talks too fast in a faintly feminine fashion and has inherited a multi-million dollar fortune from a blue-chip Republican family of Wall Street financiers.

He may be an unlikely and unprepossessing figurehead of the new American left, but he is nonetheless giving the Democratic Party establishment palpitations this summer.

The Greenwich-based businessman is mounting a ferocious challenge against three-term Connecticut Senator Joe Lieberman for the party’s nomination in a primary election. Opinion polls show him closing rapidly on the incumbent, who has been sufficiently spooked to announce this week that he will stand as an independent if he loses on August 8.

Propelling Mr Lamont forward is a shrilly vocal army of anti-war internet activists, who have been infuriated by Mr Lieberman’s support for President Bush over Iraq and what they have portrayed as his bi-partisan cosiness with Republicans on other issues.

The Democratic leadership is deeply uneasy about this contest. Not only does it threaten to end the 36-year political career of one of its biggest beasts in Washington; Mr Lamont’s challenge is also turning the national spotlight on to the party’s divisions at a time when Democrats should be presenting a united front for mid-term elections in which they have a good chance of re-taking control of one - or both - houses of Congress.

Even more disturbing for them is the prospect of the Connecticut primary becoming a model for the presidential primary elections in 2008. Howard Dean got uncomfortably close to winning the nomination last time with the help of the anti-war vote, and the subsequent fall-out did much to cloud the clarity of John Kerry’s campaign.

If the internet activists, the so-called "net-roots" can take out Mr Lieberman, they could yet wreak havoc with the presidential ambitions of Hillary Clinton, another Senator who has notably failed to renounce her earlier support for the war.

Mrs Clinton, who was recently booed and hissed at a Democratic gathering for saying she opposed setting a "date certain" for withdrawal from Iraq, was swift this week to distance herself from Mr Lieberman. She declared she would not support him if he ran as an independent and, in a nod to the party’s liberal base, added that "I believe in the Democratic Party and I believe we must honour the decisions made by Democratic voters."

Mr Lieberman, who only six years ago was a few Floridian hanging chads away from becoming vice-president, told The Times that he is being unfairly singled out. "A lot of other Democrats have a similar position to me on Iraq, but this has become a cause celebre - I’ve no idea why."

His opponents’ answer to that question is pinned to many of their lapels. They have produced a campaign badge showing Mr Lieberman and George Bush embracing after the president’s 2005 State of the Union address, along with the message: "The kiss: too close for comfort." The Senator initially tried to make light of it, saying that at least he had not kissed Mr Bush back. Now he denies there was any lip contact at all, but admits to getting little hug while the president whispered in his ear: "Thank you for being a patriotic American."

The grassroots' unhappiness with Mr Lieberman pre-dates the war. Their list of complaints against him range from his support for Mr Bush's energy bill, his flirtation with privatising social security and interest in school vouchers, as well as his support for free trade and for the role of religion in public life.

In Connecticut, they say the rot set in when Al Gore chose Mr Lieberman to be his running mate in 2000. His subsequent failed effort to win the presidential nomination in 2004 underlined their feeling he as more interested in his national profile than bread and butter issues.

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