US President Barack Obama's administration has launched an attack on Mitt Romney's values and foreign policy credentials.
Meanwhile, a fresh set of prominent Republicans have rallied behind the party's likely presidential nominee, further signs the general election is overtaking the primary season.
A Romney victory in today's Republican primary in Wisconsin could all but end the battle to pick a challenger to Mr Obama in the November election.
The ex-Massachusetts governor is predicting success in the upper Midwestern state as he shifts his campaign attacks almost solely to Mr Obama.
The president has seen re-election prospects improve due to the tough Republican nomination fight and signs the economy is in a sustained recovery.
The Obama camp did not waste time to attack Mr Romney, with both Secretary of State Hillary Clinton and Vice-President Joe Biden making scathing comments. "I think Governor Romney's a little out of touch," Mr Biden said.
"I can't remember a presidential candidate in the recent past who seems not to understand, by what he says, what ordinary middle-class people are thinking about and are concerned about."
While Mr Obama is a millionaire, Mr Romney, the former CEO of a private equity firm, would be among the nation's wealthiest presidents ever elected.
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