Mitt Romney effectively claimed the Republican presidential nomination on Tuesday, reveling in a five-state primary sweep and urging voters to help him oust President Barack Obama in November.

With wins in Connecticut, Delaware, New York, Pennsylvania and Rhode Island, according to US media forecasts, Romney essentially kicked off his general election campaign and began making his case to take over the White House.

"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 the northeastern states that are largely friendly territory to 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 6th!"

After Tuesday, Romney will still be 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 Romney as the Democratic incumbent's challenger on November 6.

"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."

The speech revolved around a common theme that Republicans have crafted about Obama: that the president has failed to turn the country around.

"Four years ago, Barack Obama dazzled us in front of Greek columns with sweeping promises of hope and change," Romney said.

"But after we came down to earth, after the celebration and parades, what do we have to show for three and a half years of President Obama?

"Is it easier to make ends meet? Is it easier to sell your home or buy a new one?" Romney asked.

"Have you saved what you needed for retirement? Are you making more in your job? Do you have a better chance to get a better job? Do you pay less at the pump?"

The Obama White House -- which has treated Romney as its main opponent for months -- quickly slammed the candidate for making a speech "full of even more distortions."

"Here is the truth: when the president took office, we were losing 750,000 jobs a month because of the failed Bush policies -- policies that Mitt Romney would bring back if elected," Obama campaign spokeswoman Lis Smith said in a statement.

"Romney may hope that Americans overlook the fact that the president took office during a global economic crisis, but he can't overlook the 4.1 million private sector jobs that have been created over the past 25 months, or the tax cut that President Obama gave to every working American."

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 next few weeks.

Gingrich, under pressure from Republicans to drop out, campaigned in the small state of Delaware where he earned some endorsements by state politicians.

It wasn't enough, and Gingrich 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.