Having won Tuesday's primary in South Carolina by a margin of 15%, and polled second in three of the other six states that voted that night, he has emerged as the main rival to John Kerry.

A breakdown of the votes cast showed that Mr Edwards did well among black and white voters as well as moderates, independents, women and rural voters.

But his failure to win anywhere other than South Carolina has left him vulnerable to the charge that he is a "regional candidate" who performed well only because he is a native and serves as senator for neighbouring North Carolina.

Others agree that while Mr Edwards' victory has kept him in the race, he will have to prove that he can appeal to voters north of the Mason-Dixon line.

"I think it is important for Edwards to show what he can do beyond the south," Jack Fleer, political scientist at Wake Forest University told Columbia's State newspaper.

Mr Edwards' insistence on running only a positive campaign has made it difficult for him to draw distinctions between himself and other candidates, and he says he plans to compete seriously in Michigan, a northern industrial state.

But, as the race narrowed yesterday and his campaign headed for Tennessee and then Virginia - which hold primaries next Tuesday - the battle lines became obvious.

Mr Edwards said he aimed to emphasise his working-class roots as the son of a mill worker and compare them with Mr Kerry's wealthy upbringing, which included private schooling in Switzerland.