The US presidential race has been left wide open after Hillary Clinton and John McCain both rebounded to win victories in the New Hampshire primary.
It appeared that Mrs Clinton's campaign benefited from a surge among women voters, while fewer young voters turned out for Democratic rival Barack Obama.
John McCain appealed to New Hampshire's independent voters but his victory leaves no clear Republican frontrunner.
Attention will now focus on Michigan, Nevada, South Carolina and Florida.
Candidates are aiming to build momentum before more than 20 states hold polls on 5 February, known as Super Tuesday.
Democratic Senator Hillary Clinton's victory defied pundits and pollsters alike. Correspondents say the result leaves Mrs Clinton and Barack Obama essentially tied.
They take their fight now to Nevada where Mr Obama has just won an important union endorsement and, crucially, to South Carolina, where black people make up half of the Democratic primary electorate.
Democratic candidate John Edwards might also poll well in South Carolina where he was born, splitting the anti-Clinton vote, says BBC North America editor Justin Webb.
Mr Edwards, who came third in the contest, reminded supporters in Manchester, New Hampshire, that there were "48 states left to go".
On the Republican side, the win for John McCain also throws a spotlight on South Carolina where the Iowa victor, Mike Huckabee, could rebound, buoyed by evangelical Christians who have been his firmest supporters, says our correspondent.
Mrs Clinton's rebound echoed that by her husband, former President Bill Clinton, who in New Hampshire's primary in 1992 called himself the "comeback kid" when his faltering White House bid was resurrected by a strong, second-place finish.
Clinton aides said her win may be down to an extraordinary moment on Monday when she appeared close to tears as she talked about how much public service meant to her.
Exit polls suggested female voters, who deserted Mrs Clinton last week in Iowa, and registered Democrats helped sweep her to victory, overcoming Mr Obama's advantage among New Hampshire's independent voters.
The 46-year-old first-term senator from Illinois had gone into the vote with leads of up to 13 points in opinion polls after his impressive win in last week's Iowa caucuses, having trailed Mrs Clinton for much of the campaign.
In the Republican race, Arizona Senator McCain rode to victory, taking 37% of the vote to beat his higher spending rival Mitt Romney into second place with 32%.
Ex-Arkansas governor and Baptist minister Mike Huckabee and former New York mayor Rudy Giuliani came in with 11% and 9% of the vote for the Republicans.
Mr McCain, a 71-year-old former Vietnam prisoner of war, thanked his supporters, saying: "I'm past the age where I can claim the noun 'kid', no matter what adjective precedes it. But tonight we sure showed them what a comeback looks like."
He had been the frontrunner until his campaign ran out of steam last year when his support evaporated, analysts said, because of his support for the Iraq war.
Meanwhile, Mr Romney faces a tough challenge to bounce back from his Iowa and New Hampshire defeats, states where he far outspent his opponents.
Mr Giuliani - who analysts say has yet to fully enter the fray - has focused his efforts on Florida's 29 January primary.
Mr Huckabee will take heart from his third place finish in a state where he was never expected to do well, say correspondent.