The former Massachusetts governor was projected to win in Connecticut, Delaware, New York, Pennsylvania and Rhode Island.
With a big lead in delegates and his nearest competitor out of the race, Romney told a cheering victory rally in New Hampshire, "A better America starts tonight."
"The last few years have been the best that Barack Obama can do, but it's not the best America can do," Romney said. "Tonight is the beginning of the end of the disappointments of the Obama years, and it's the start of a new and better chapter that we will write together."
Speaking to CNN's Piers Morgan after Romney's speech, Obama 2012 press secretary Ben LaBolt said, "The fact is a better title for Gov. Romney's speech tonight, [rather] than 'A Better America,' should have been 'Back to the Future,' because he's proposing the same economic policies that got us into the economic crisis in the first place."
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