Mitt Romney laid claim to a fiercely contested Republican presidential nomination after sweeping five primary contests and immediately set the tone for the general election campaign by attacking President Barack Obama over his handling of the economy.

After struggling for months to prevail over unexpectedly persistent rivals, the Republican nominee-in-waiting was eager to refocus his efforts on the campaign against Obama.

"Tonight is the start of a new campaign," the former Massachusetts governor said on Tuesday night as he celebrated his primary victories with a blast at Obama as a man whose time in office has been marked by "false promises and weak leadership" in a time of economic struggle.

He delivered his remarks to a national television audience from New Hampshire, the state where he won his first primary of the campaign and one of about a dozen states expected to be battlegrounds in the campaign for the White House.

Romney won primary victories on Tuesday in Connecticut, Rhode Island, Delaware, Pennsylvania and New York in the first contests since his chief rival, former Pennsylvania Senator Rick Santorum, conceded the race.

Romney was eager to leave the nominating campaign behind.

"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," he said.

Romney planned to intensify fundraising efforts on Wednesday and Thursday to prepare for what may be the most expensive presidential contest in the history of American politics. The presumptive Republican nominee has at least six fundraising events in two days in New York and New Jersey. Romney's campaign had only about $10 million in the bank at the end of March, according to federal filings. All told, Obama reported more than $104 million in his account, having already spent nearly $90 million on the general election. Election Day is Nov 6.

Six months before the election, opinion polls show the economy to be the top issue by far in the race.

The same surveys point toward a close contest, with several suggesting a modest advantage for Obama.

Obama won the presidency in 2008 in the midst of the worst recession since the Great Depression of the 1930s, and since then economic growth has rebounded slowly and joblessness has receded gradually while housing prices have continued to drop in many areas of the country.