Almost at the instant election results began to emerge on Tuesday night, people started joking about Groundhog Day, the annual event in the small town of Punxsutawney, Pennsylvania, in which local elders determine whether winter is ending early by measuring the shadow on a groundhog.

"Six more weeks of winter!" may indeed have been an apt reading of Hillary Clinton's upset victories over Barack Obama in Texas and Ohio, since the focus of this epic but grinding nomination race will now shift to the chilly state of Pennsylvania, which holds its primary election on 22 April.

With the exception of the nominating contests in Wyoming on Saturday and Mississippi next Tuesday, that means the two candidates will spend most of their next seven weeks criss-crossing Pennsylvania in what campaign officials readily concede will be a replay of the Iowa caucuses that kicked off this race just eight weeks ago.

The candidates spent much of 2007 working the mid-western state in the build-up to its January 3 vote. "This is going to be a long and grinding slog," said a senior Obama official. "We are going to get almost as well acquainted with Pennsylvania as we were with Iowa."

The race may also start to turn relentlessly negative. Mrs Clinton's victory in Ohio, Texas and Rhode Island on Tuesday was preceded by a week-long barrage of hammer blows and negative advertisements aimed at undercutting Mr Obama's reputation for transparency, his capacity to be commander-in-chief and his ability to beat John McCain in a general election.

Given the apparent success of these tactics - more than two-thirds of voters in Ohio who made their mind up in the preceding 72 hours opted for Mrs Clinton - the Obama camp is bracing itself for much more. Mr Obama will also have to contend with further questions from the Clinton campaign and the media on his ties to Tony Rezko, the Chicago businessman whose criminal trial on charges of corruption began on Monday.

Nobody has suggested that Mr Obama has done anything wrong and his campaign has returned about $150,000 (€98,000, £75,000) in donations arranged by Mr Rezko. But the Clinton campaign held repeated conference calls with the media to berate them for not scrutinising his ties with Mr Rezko more closely. On his campaign aircraft on Tuesday shortly before the results came in, Mr Obama tried to put a positive gloss on what many see as the end of his long media honeymoon.

"Look, they have run a pretty negative campaign over the past couple of weeks," he told reporters on board "Obama One". "My theory is that withstanding some of the attacks that have been coming our way will just make us stronger. It's good preparation. It's like training camp if I end up being the nominee."

The outlook for Mrs Clinton, whose victories in Ohio and Texas mark her third comeback after having twice before been written off by the media, may not be quite as rosy as they seemed at first blush. Mrs Clinton's victories ought to give her new fundraising powers to try to match Mr Obama's formidable online money machine, which raised $50m in February to her $35m.

However, even if Mrs Clinton beats Mr Obama by a handsome margin in Pennsylvania, she is still likely to end the nominating contest on June 7 - when Puerto Rico goes to the polls - with fewer elected delegates than Mr Obama. Indeed, her net gain of 26 delegates on Tuesday could well be wiped out over the next five days in Wyoming and Mississippi, both of which tilt heavily towards Mr Obama.

That means the focus over the next few weeks will once again turn to the 796 unelec-ted "superdelegates", whose support looks essential to tipping one or other of the candidates over the magic threshold of 2,025 to win the nomination. At least 500 of the superdelegates - most of whom are elected Democratic officials - are undecided and there is nothing to stop the other 296 from shifting their allegiance.

The odds that the race will now continue right up until August 25, when the Democratic convention takes place in Denver, have risen sharply. And the odds that party notables such as Al Gore, Jimmy Carter, Nancy Pelosi or John Edwards can intervene before then to persuade Mrs Clinton to step aside in the interests of party unity have sharply diminished.

Beyond that, almost anything can happen between now and Pennsylvania to shift the race one way or the other. Damaging revelations could emerge about Mr Obama from the Rezko trial. Mrs Clinton's tax returns, which she has pledged finally to release in April, could provide new material for investigative journalists. Or one or other of the candidates could make a game-changing gaffe.

"We are now in completely uncharted waters," says Simon Rosenberg, the president of the New Democratic Network, a liberal think-tank in Washington. "Any pundit who says that they know what is going to happen is taking a punt along with everyone else."

To take one small punt, however, the race is likely to continue along negative lines. In the 1993 comedy Groundhog Day , the protagonist experiences the same day over and over again before changing character and waking up to a new day. The candidate who proves best able to rejuventate the campaign over the next few weeks will stand the best chance of a happy ending.

By Edward Luce, Financial Times, March 6 2008