Barack Obama and Hillary Clinton continued to trade blows at every step as they awaited the results of elections in Wisconsin and Hawaii before heading into the next - possibly decisive - stage of the Democratic presidential race.

Attention is focusing on Wisconsin, which offers 74 convention delegates and an early test of who can galvanise support in industrial states ahead of an even more crucial contest in Ohio next month.

There are also 20 delegates at stake in Hawaii's caucuses, a state where Mr Obama grew up but neither candidate has campaigned in person.

After eight successive defeats, Mrs Clinton has made a belated effort to stem Mr Obama's tide in Wisconsin where a big slice of the electorate are the white and working class - groups which have largely voted for her in other elections.

She has spent time in the state over recent days, as well as running an aggressive TV advertising campaign attacking her opponent's healthcare policy and his reluctance to engage her in debate.

Mr Obama's aides suggested the state - which contains few of the black voters who have boosted him elsewhere - should have been natural Clinton territory.

But her campaign points out recent polls showing he had established a clear advantage. An aide said: "We have been working hard and will do as well as we can. But this is a state where the Obama campaign itself has predicted victory."

Indeed, Wisconsin also has numbers of progressive liberals who have skewed towards him elsewhere and exit polls are suggesting roughly one quarter of voters are independents - a group which also leans towards him.

The economy was a key issue, according to the survey. Seven in 10 Democratic primary voters said US trade with other countries winds up costing jobs in Wisconsin. Fewer than one in five said it creates more jobs than it loses.

Turnout was said to be high - around 35 per cent - despite freezing weather. Even as voters went to the polls Mrs Clinton's camp were ramping up charges of plagiarism against Mr Obama. His aides, in turn, were pouring sarcasm over her newly-populist stance on trade deals which are said to have damaged US manufacturing employment.

Mr Obama has admitted he should have credited Massachusetts Governor Deval Patrick with lines he used to rebut criticism from Mrs Clinton that he was all talk - and no substance. But his campaign claims Mrs Clinton is "grasping at straws to try and create some momentum".

But she has responded by saying: "If your whole candidacy is about words, those words should be your own - that's what I think." On Tuesday, her campaign highlighted a second alleged case of copying the Massachusetts governor.

Last year Mr Obama said: "I am not asking anyone to take a chance on me. I am asking you to take a chance on your own aspirations." Mr Patrick, a prominent supporter, has used exactly the same words in 2006.

Howard Wolfson, Mrs Clinton's spokesman, said: "We are seeing a pattern here. Senator Obama does not have long record of public service. He is running on powerful oratory...So when he is found to have lifted passages from another elected official it is significant."

Mr Obama said that such attacks from “the Clinton machine” had prepared him for a Republican onslaught in a general election.

But hours later, his campaign got a taste of a general election campaign with clashes between two putative First Ladies.

On Monday, Michelle Obama had said: "Let me tell you something — for the first time in my adult lifetime, I am really proud of my country. And not just because Barack has done well, but because I think people are hungry for change."

This morning, John McCain's wife, Cindy, responded: “I am proud of my country. I don’t know about you, if you heard those words earlier — I am very proud of my country.”

The Clinton campaign is saying it is still in a good position in its two "must win" states on March 4 Texas and Ohio.

Robby Mook, her state director in Ohio promised an "aggressive campaign" while his counterpart in Texas, Ace Smith, said "we're going to concede absolutely nothing" to Mr Obama.

Both candidates have put increasing emphasis on economic issues in recent days ahead of the crunch elections in these two states where many Democrats are blue collar workers who fear further job looses.

Mrs Clinton has issued a 12-page manifesto of her economic policies for problems ranging from high oil prices to mortgage foreclosures. She launched a TV advert in Ohio aimed at low incomes workers saying: "You’re often overworked, underpaid, and sometimes overlooked."

Mr Obama has also focused on the economy, promising to crack down "on lenders who take advantage of working families" and end tax breaks for companies which move jobs overseas. His campaign circulated a memo contrasting Mrs Clinton's support for the Nafta deal with her promise now for a "time-out" on such trade agreements. "No one knows when this time-out will end. Maybe after the election," it said.

The endorsement of John Edwards, who dropped out of the Democratic presidential race earlier this month but still carries great sway among white lower-income voters, could yet prove critical in deciding the winner.

Mr Obama, who flew down to North Carolina to see him on Monday, said he did not expect any immediate announcement. "I think he's still mulling things over," he added:

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