Mitt Romney's path to the Republican nomination has been cleared further as former threat Newt Gingrich began taking steps to shut down his debt-laden campaign and endorse him.
Mr Romney swept five state primaries 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".
Mr Gingrich had hoped for a stronger showing. His spokesman RC Hammond said the former House speaker spoke with Mr Romney and had started planning an event to endorse him next week. Mr Gingrich called on the party to unite behind Mr Romney.
Mr Romney is widely expected to emerge from the Republican nominating convention in August as the party's challenger to Mr Obama in November.
The former Massachusetts governor is attacking the president 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," Mr Romney said in a victory speech. "This does not have to be. It is the result of failed leadership and of a faulty vision."
Mr 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 he is better equipped to handle.
Nevertheless, polling shows Mr Obama with a far higher rating and leads in the dozen states expected to be decisive battlegrounds in the campaign for the White House.
Mr Romney won primary victories 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.
He plans to intensify fundraising efforts 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.