AP

Mitt Romney's path to the Republican nomination has cleared further as former threat Newt Gingrich begins taking steps to shut down his debt-laden campaign and endorse him.

Romney swept five state primaries on Tuesday and immediately turned his sights on President Barack Obama, telling voters he would save them from four more years of what he called Obama's "false promises and weak leadership."

Gingrich had hoped for a stronger showing on Tuesday. Spokesman RC Hammond said the former House speaker spoke with Romney on Wednesday and had started planning an event to endorse him next week. Gingrich on Wednesday called on the party to unite behind Romney.

Romney is widely expected to emerge from the Republican nominating convention in August as the party's challenger to Obama in November.

The former Massachusetts governor is attacking Obama over his handling of the US economic recovery from the steepest downturn since the 1930s Great Depression.

"As I look around at the millions of Americans without work, the graduates who can't get a job, the soldiers who return home to an unemployment line, it breaks my heart," Romney said in a victory speech Tuesday. "This does not have to be. It is the result of failed leadership and of a faulty vision."

Romney, who shed his closest Republican rival when Rick Santorum left the race this month, is focused on the still-weak economy, the foremost issue among voters and one that polls show Americans believe Romney is better equipped to handle.

Nevertheless, polling shows Obama with a far higher favourability rating and leads in the dozen states expected to be decisive battlegrounds in the campaign for the White House.

Romney won primary victories on Tuesday in Connecticut, Rhode Island, Delaware, Pennsylvania and New York. He must now convince independent voters he is the best man to take over the White House.

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 US politics. He has at least six fundraising events in two days in New York and New Jersey.

Romney's campaign had only about $US10 million ($A9.7 million) in the bank at the end of March, according to federal filings. Obama reported more than $104 million in his account, having already spent nearly $90 million on the general election.

Obama campaigned on Wednesday at the University of Iowa, promoting actions his administration has taken to help low-income and middle-class Americans.

"Our businesses have added more than four million jobs over the past two years, but we all know there's still too many Americans out there looking for work or trying to find a job that pays enough to cover the bills and make the mortgage," the president said.

He is focusing on young voters, an integral part of his political base along with women and Hispanics.

Obama, unchallenged for the Democratic nomination, has a head start in organising, fundraising and other elements of the general election campaign.

Already, he and aides are working to depict Romney and Republicans as pursuing new tax breaks for the wealthy while seeking to cut programs that benefit millions of victims of the recession as well as other lower-income Americans.