US president Barack Obama's administration has launched a multi-front assault on Mitt Romney's values and foreign policy credentials.
Meanwhile, a Romney victory in Tuesday's Republican primary in Wisconsin could all but end the battle to pick a challenger to Mr Obama in the November election.
The former Massachusetts governor is predicting success in the upper Midwestern state as he shifts his campaign attacks almost solely to Mr Obama and away from fellow Republicans.
The President has seen his prospects of re-election improve somewhat as a result of the protracted Republican nomination fight and signs the economy is in a sustained recovery.
Even as Mr Romney looks ever more likely to win the nomination, a defiant Rick Santorum vowed to stay in the race, saying he would not give up just because the Republican party establishment believed voters "need Mitt Romney shoved down their throats". The Wisconsin vote will be Mr Santorum's last chance to prove his strength in the US heartland, where he has said he can challenge Mr Obama but where Mitt Romney has beaten him consistently.
The Obama camp was not wasting any time in taking the attack to Mitt Romney, with both secretary of state Hillary Clinton and vice president Joe Biden going on the attack. "I think Governor Romney's a little out of touch," Mr Biden told CBS' Face the Nation. "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."
Mr Obama's team on Sunday also seized on Romney's foreign policy inexperience. Mr Biden said Barack Obama was "stating the obvious" when he told Russian president Dmitry Medvedev that he would have more latitude on missile defence after the November general election. The two presidents did not realise the exchange, during a meeting in Seoul, South Korea, last weekend, was being picked up by an open microphone.
Mr Romney called it "alarming" and part of a pattern of "breathtaking weakness" with America's foes. He asked what else Mr Obama would be flexible on if he were to win a second term.
"Speaking of flexible, governor Romney's a pretty flexible guy on his positions," Mr Biden said. Mitt Romney's Republican opponents have accused the former Massachusetts governor of "flip-flopping" on issues such as health care reform and abortion.
Mrs Clinton seized on Mr Romney's comment that Russia is America's "Number one geopolitical foe", calling the statement "dated" and suggesting there were more pressing matters of concern in global affairs. "I think it's somewhat dated to be looking backwards instead of being realistic about where we agree, where we don't agree," she told CNN Sunday. "He just seems to be uninformed or stuck in a Cold War mentality," Mr Biden added. "It exposes how little the governor knows about foreign policy."