Kerry's seemingly unstoppable drive faces toughest challenge in the south

John Kerry seemed almost unstoppable in his drive for the Democratic presidential nomination after two more big victories at the weekend, whileHoward Dean, the former front-runner, suffered a humiliating new blow as a major union withdrew its earlier endorsement for his faltering candidacy.

Mr Kerry's latest successes in Michigan and Washington state mean that he has won nine of the first 11 primaries and caucuses, showing strength in every corner of the country except the south. The Massachusetts senator appeared headed for another victory last night in the New England state of Maine, where polls showed him with a solid lead in the run-up to yesterday's caucuses.

Mr Kerry, in classic front-runner style, has almost ceased to refer to his rivals in his public speeches. Instead, he is concentrating his fire on the man who - barring some shock disclosure or drastic misstep on his own part - he seems set to meet in the autumn general election. Mr Kerry, fending off charges from the White House that he is a typical "Massachusetts liberal" out of touch with popular feelings, said it was George Bush who was running "extremist" policies. He told a candidates' dinner in Richmond, Virginia, on Saturday: "Democrats will not run from a fight about who represents mainstream America."

Mr Kerry won a massive 52 per cent of the vote in Michigan, the first major industrial state to have a say in the nomination battle, more than three times the share of his nearest rivals, Mr Dean and Senator John Edwards of North Carolina.

In liberal Washington state, fertile soil for Mr Dean, the former Vermont governor fared better, winning 30 per cent. But that was eclipsed by the 49 per cent who backed Mr Kerry, in a state where, until his astonishing comeback at last month's Iowa caucuses, he had made almost no impression.

The battle now shifts to two southern border states, Virginia and Tennessee, which hold primaries tomorrow. These are shaping up as an elimination round between Mr Edwards and retired General Wesley Clark, whose Arkansas childhood makes him the other candidate with southern connections. Failure to win either state could prove fatal for both of them, even though Mr Edwards seemed to be preparing for precisely that outcome by saying he needed no more than "strong second" places. And that may well be what happens.

Polls suggest that Mr Kerry has surged into the lead in both Virginia and Tennessee. The contest could be virtually decided there and then should he achieve a southern double, three weeks before the so-called "Super Tuesday" on 2 March, when California, New York and Ohio are among 10 states holding primaries with more than 1,100 delegates at stake; more than half the total required for the nomination.

But Mr Dean may have exited the race well before that. He is gambling all on winning the Wisconsin primary on 17 February. But his cause was further damaged at the weekend when AFSCME, the local government employees union, announced it was withdrawing its support, and heightening the sense that the former Vermont governor's candidacy is unravelling by the day.

Mr Dean plans a $1m-plus (£550,000) advertising blitz in Wisconsin but polls suggest he is currently running no better than fourth in the upper mid-Western state.