Mitt Romney effectively claimed the Republican presidential nomination as he reveled in a five-state primary sweep and urged voters to help him oust President Barack Obama in November.
With wins Tuesday in Connecticut, Delaware, New York, Pennsylvania and Rhode Island, according to US media forecasts, Romney essentially kicked off his general election campaign after months of tangling with Republican rivals.
"Tonight is the start of a new campaign," Romney told ecstatic supporters in New Hampshire -- scene of his first Republican primary victory back in January and a potentially pivotal general election battleground.
"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."
He trounced former House speaker Newt Gingrich and congressman Ron Paul, the two remaining Republicans in the race, in northeastern states that are largely friendly territory for Romney, a former governor of Massachusetts.
Romney basked in the glow of the clean sweep, and effectively staked his claim to the nomination.
"After 43 primaries and caucuses, many long days and not a few long nights, I can say with confidence -- and gratitude -- that you have given me a great honor and solemn responsibility," Romney told supporters.
"Together, we will win on November 6!"
Romney is still short of the 1,144 delegates needed to be crowned the official nominee at the Republican convention in late August, but most campaign watchers are treating him as the Democratic incumbent's challenger.
"The nomination struggle is over," G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin & Marshall College in Pennsylvania, told AFP.
After Tuesday, Madonna said, "he stays on that message. It's the general election day in and day out."
Romney sought to assure struggling Americans -- "the thousands of good and decent Americans I've met who want nothing more than a better chance" -- that he was the candidate ready to fight for a fair and improving economy.
"To all of you, I have a simple message: hold on a little longer. A better America begins tonight," he said, reprising the central theme of the Republican campaign -- that Obama has failed to turn around the US economy.
The Obama White House -- which has treated Romney as its main opponent for months -- also appeared to formally kick off its general campaign.
"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."
Obama's supporters have warned that Republicans would return to the policies of the George W. Bush administration, which it says favored the wealthy, worsened inequality and caused the 2008 economic collapse.
"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," LaBolt said.
Obama meanwhile swung through North Carolina and Colorado, two key battlegrounds in the general election, where he called on Congress to act to prevent interest on federal student loans from doubling in July.
Romney has 683 delegates, compared with 141 for Gingrich and 84 for Paul, according to a tally by RealClearPolitics.com.
Rick Santorum has 267 delegates, but he bowed out of the race two weeks ago.
Santorum has declined as of yet to endorse Romney, but said Tuesday he was hoping to meet with his former rival in the coming weeks.
"I will support the nominee of our party," Santorum told CNN during an interview in which he repeatedly declined to directly endorse Romney.
"If he's the nominee I'm going to do everything I can to make sure that he defeats Barack Obama... That's why I got into this race."
Gingrich, under pressure from Republicans to drop out, campaigned in the small state of Delaware, where he earned some local endorsements but finished with less than half as many votes as Romney.
While Gingrich refused to quit, he acknowledged a reassessment was in order.
"Over the next few days we're going to look realistically at where we're at," Gingrich reportedly said.