Voting is under way in the keenly anticipated Connecticut primary elections, where Senator Joseph Lieberman, one of America's best known and most experienced Democrat politicians, faces potential defeat at the hands of a political newcomer because of his unstinting support for the Iraq war.

The contest, in which Connecticut's nearly 700,000 registered Democrat voters will decide their candidate for the Senate in November's midterm elections, is being watched across the country as a guide to the level of anti-war feeling among the party faithful, and an unsettling referendum on the direction of the party as a whole.

Ned Lamont, a millionaire owner of a television company, has startled the established leaders of the Democrat party by leading Mr Lieberman, a senator for 18 years, throughout the campaign.

Mr Lamont's message, which in polls last week gave him a 13-point lead over Mr Lieberman, has been simple: to "change course" in the Senate by pulling frontline American troops out of Iraq.

Mr Lieberman, who is on the hawkish side of Democratic foreign policy and was once rumoured to be about to join George Bush's cabinet, has been damaged by his consistent support for the American invasion of Iraq in 2003. He has also been photographed hugging the President.

The directness of the challenge to Mr Lieberman, an immensely respected politician who was Al Gore's running mate in the 2000 presidential election, has prompted the senator to admit that he is in "the political battle of his life" and to call in favours from the celebrities of the party. Bill Clinton has visited the state to campaign on his behalf.

Like other Democrats close to the centre, including Hillary Clinton, who are keen to try to win moderate and conservative voters during the midterm elections and the presidential race in 2008, Mr Lieberman has warned voters that a slide to the anti-war left plays into the hands of the Republicans who are keen to split the party.

"They are anxious to say the left wing is taking over, the anti-security wing," Mr Lieberman said last night.

In the last few days of the campaign, Mr Lieberman's counter-attacks - he says Connecticut needs a senator, rather than a message - have appeared to pay off. Last night, as the candidates entered a frenetic last 24 hours of rallies, television appearances and phone calls to voters, a poll showed him just six points behind Mr Lamont.

The intensity of the electioneering was shown in reports in The New York Times that Mr Lieberman's campaign was trying to poach members of Mr Lamont's staff by nearly doubling their pay to $150 a day. Mr Lamont has so far spent $3 million on his campaign.

Today voters were divided, some citing Mr Lieberman's experience as the reason to continue supporting him, others saying he had become too close to the Bush Administration.

"I’m completely for Lamont because of the war issue. I’m totally disgusted with Lieberman and his positions. I think he sold us out," William Clement, a 57-year-old teacher, said after casting his vote for Mr Lamont in the town of Hartford.

Mr Lieberman voted in his hometown of New Haven before making six stops across the state today. He planned to wait for the results in a hotel in Hartford after the polls close at 8pm local time (0100 BST).

Mr Lamont, by contrast, said he intended to spend the day making phone calls and giving interviews to the local press. Mr Lieberman has vowed to run as an independent candidate in the mid-term elections if he is defeated. Mr Lamont has said he will back whoever wins.

The race is one of five primary elections taking place across America today. The others are Colorado, Missouri, Michigan and Georgia, where Congresswoman Cynthia McKinney, who punched a police officer on Capitol Hill earlier this year, faces a challenge for her Democratic nomination.

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