President Barack Obama's camp greeted Mitt Romney's coronation as Republican nominee, warning he would turn America "Back to the Future" to crisis-causing economic policies.
"Mitt Romney has spent the past year out on the campaign trail tearing down the president with a negative message that even Republicans who have endorsed him have criticized," said Obama campaign spokesman Ben LaBolt.
"This marks the end of that monologue. Now he must put his record and his agenda next to the president's."
Romney swept primaries in Connecticut, Delaware, Pennsylvania and Rhode Island, according to US media projections, solidifying his already tight hold on the Republican nomination to take on Obama in November's election.
Obama spent the evening before a huge crowd of up to 11,000 students in the swing state of Colorado, without directly mentioning his foe, but sending a message that he plans to rebuild his dominance of the key youth vote.
He had earlier told students at the University of North Carolina that he had walked in their shoes, empathizing with middle class struggles by saying he had only paid off his student loan debt eight years ago.
The president, the incumbent leader on whom voters will cast judgment in November, portrayed himself as more in tune with Americans struggling in tough economic times, than politicians from dynastic political families.
"We didn't come from well to do backgrounds, we didn't have famous families," Obama said, perhaps in an oblique reference to Romney and his father George Romney, a 1960s presidential candidate and Michigan governor.
Obama's aside also recalled the tone of his 2008 campaign when he portrayed himself as a breath of fresh air after years of politics dominated by the Clinton and Bush families.
LaBolt used his statement to set the terms of the election the Obama campaign would like to fight -- on a vision of "fair shot" American economy, rather than on Obama's checkered economic record.
"The President brought the economy back from the brink of another Depression, bet on American workers to spur the comeback of the American auto industry and American manufacturing, kept his promise to end the war in Iraq and refocus on Al-Qaeda and fought every day to build an economy where hard work paid off and responsibility are rewarded," LaBolt said.
"The title for Governor Romney's speech tonight should have been 'Back to the Future' because he has proposed a return to the same policies that got us into the economic crisis in the first place."
"Governor Romney believes that showering the wealthiest Americans with special giveaways will make the middle class thrive.
"We have tried those policies before. They didn't unleash growth, they didn't spur job creation and they didn't boost the middle class."
Romney, in a speech in New Hampshire effectively claimed the mantle of Republican nominee after 43 primaries and caucuses and his earlier struggles to win over the conservative grass roots.
"You have given me a great honor and solemn responsibility," Romney said.
"Together, we will win on November 6th!"