US Senator John Kerry squeezed past John Edwards to win the Wisconsin primary for the Democratic nomination to challenge George W Bush for the White House, leaving former front-runner Howard Dean considering backing one of his rivals.
Clinging to a five-point lead, Mr Kerry said: "A win is a win."
Senator Edwards, his dream of a head-to-head matchup now a reality, declared: "We'll go full-throttle to the next group of states."
He promised to campaign in California, New York, Ohio and seven other states holding primaries or caucuses on "Super Tuesday" March 2. At stake will be 1,151 delegates, more than half the total needed to claim the nomination.
Taking advantage of Wisconsin's open primary rules, one in 10 voters were Republicans and about 30% were independents. Those voters backed Mr Edwards.
Mr Edwards's breakout was fuelled by the highest Republican turnout of the primary season and voters who made their decision in the last week. His deepest support was in the Republican suburbs of Milwaukee.
"That's been happening in other primaries, too," Mr Edwards told The Associated Press in an interview. "Republicans who would consider voting Democratic and independents are the people we have to win over to win the general election. That's why I'm the best candidate to take on George Bush."
Mr Kerry held a wide lead in pre-election polls, but the surveys did not fully reflect voter sentiments after a state-wide debate on Sunday, Mr Edwards's criticism of his main rival's free-trade policies and two newspaper endorsements for Mr Edwards. Nor did the polls take into account 11th-hour criticism of Mr Kerry from President Bush's re-election team.
"We underwent a lot of Republican attacks the last week," Mr Kerry told the AP. "Notwithstanding those attacks, we showed we can fight back.
"We're winning in every state across the country. We're going to win the nomination."
Kerry won 15 of the 17 elections - seven by nearly half the vote - on the East and West coasts, in the Midwest, the Great Plains and the Southwest. He remains the undisputed front-runner, flush with money and momentum.
But the Edwards surprise ended any hope for a quick conclusion to the race and earlier-than-ever general election planning. A poor second-place showing would have crippled Mr Edwards's campaign.
Mr Dean ignored pleas to give up the fight. "We are not done," he told his supporters, even as his own advisers were saying his campaign for the presidency was effectively over. He headed home to Vermont to regroup, in search of a way to convert his political network into a movement that helps elect Democrats.
Senior advisers said Mr Dean was considering dramatically scaling back his campaign with no hope of winning the presidency. He was just as likely to cede the nomination and, with hopes of becoming a kingmaker, endorsing a rival.
Mr Dean called both Mr Edwards and Mr Kerry to discuss his next step, sources in all three campaigns said. The conversation with Mr Edwards was said to be warm and friendly.
Advisers said Mr Dean believes his fund-raising prowess could help reshape the race if he throws his support behind Mr Edwards. But they did not rule out Mr Dean endorsing Mr Kerry, a move they said could seal the nomination for the Massachusetts senator.
With 99% of the precincts reporting, Mr Kerry had 40%, Mr Edwards 34%, Mr Dean 18%, Congressman Dennis Kucinich 3% and the Reverend Al Sharpton 2%.