The campaign now heads towards a crunch date on March 4 when Mrs Clinton needs to win both Texas and Ohio to keep her White House hopes alive.
Mr Obama's victory in Wisconsin - by a wider than expected margin of 56 per cent to 41 per cent - increases the likelihood that a defeat for Mrs Clinton in either of these key states next month will prove to be decisive in determining the nomination.
She now trails Mr Obama by more than 150 pledged delegates and will need to win overwhelmingly in Ohio and Texas, as well as Pennsylvania on April 22, to close the gap.
As in the Potomac primary a week earlier, Mr Obama made significant inroads last night into her base support among women, union and white lower-paid voters while maintaining huge leads over among independents and younger people.
Mr Obama's aides yesterday suggested that Wisconsin - which contains few of the black voters who have boosted him elsewhere and was not a caucus contest of the type he has usually won - should have been natural Clinton territory. One described it as a "no excuses primary" for her.
The results of today's caucuses in Hawaii - a state where Mr Obama spent much of his youth - did not offer Mrs Clinton any comfort either; she took only 24 per cent of the vote compared to 76 per cent for the Illinois senator.
Mr Obama appeared in front of another massed rally at Houston when he was declared the Wisconsin winner last night. "Houston, I think we achieved lift-off here," he said to wild cheers from a crowd of around 20,000 people.
His speech began in the middle of Mrs Clinton's own address in Youngstown, Ohio, prompting TV stations to abandon coverage of her in favour of Mr Obama. This was immediately seen as a sign that his campaign is now ready to "big foot" his formerly frontrunning rival.
Mrs Clinton notably failed to congratulate Mr Obama in her speech and - although aides later said they spoke briefly by telephone - the animosity between the two campaigns appears to have hardened in recent days during which they have both run negative TV advertising attacks.
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She sought to shift voters' attention away from the excitement and euphoria surrounding Mr Obama, saying: "This election is not about me or my opponent. It's about you. Your lives, your dreams, your future."
Instead, Mrs Clinton said voters needed to focus on the real choice they faced - beyond having the first woman or the first African American Democratic nominee.
"Both Senator Obama and I would make history. But only one of us is ready on day one to be commander-in-chief, ready to manage our economy, and ready to defeat the Republicans. Only one of us has spent 35 years being a doer, a fighter and a champion for those who need a voice.
"That is what I would bring to the White House. That is the choice in this election. It's about picking a president who relies not just on words - but on work, hard work, to get America back to work. Someone who's not just in the speeches business - but will get America back in the solutions business."
Mr Obama responded in his own speech, saying change "will take more than big rallies, it's going to require more than rousing speeches, it will also require more than policy papers and positions and websites, it is going to require something more".
He added: "The problem that we face in America today is not a lack of good ideas. It's that Washington has become a place where good ideas go to die."
His speech suggested he was already looking ahead to a general election battle against the presumptive Republican nominee, John McCain, saying that America did not need the "same old folks" running Washington.
But Mr McCain, who cantered past the fading challenge of Mike Huckabee to win the Republican primary in Wisconsin last night, made thinly veiled swipes in Mr Obama's direction and "the confused leadership of an inexperienced candidate".
In a speech in Columbus, Ohio, he said: "I will fight every moment of every day in this campaign to make sure that Americans are not deceived by an eloquent but empty call for change."
© 2008 The Times Online