The former Massachusetts governor was polling at least 56 percent in each of his victories in Connecticut, Delaware, New York, Pennsylvania and Rhode Island, while election returns showed. Rep. Ron Paul of Texas, former U.S. House Speaker Newt Gingrich of Georgia and former U.S. Sen. Rick Santorum of Pennsylvania, who has left the race April 10, split the remaining votes.
In a victory speech he delivered while in Manchester, N.H., Romney made it clear his campaign is now against the Democratic incumbent, President Barack Obama.
"Thank you America. I can say with confidence and gratitude that you have given me a great honor and solemn responsibility and together we are going to win on Nov. 6," Romney said.
Romney, in advance remarks provided reporters, indicated his campaign will focus on showing voters he is the candidate best-suited to grow the United States' middle class, CNN reported.
"I see an America with a growing middle class, with rising standards of living," Romney said in the prepared remarks. "I see children even more successful than their parents -- some successful even beyond their wildest dreams -- and others congratulating them for their achievement, not attacking them for it."
Romney went into the day's elections with an estimated 695 of the 1,144 delegates he needs to cinch the nomination, with Gingrich at 141 and Paul at 72, CNN said. There were 224 delegates up for grabs in the five states Tuesday.
Tuesday was the first primary day since Romney's closest challenger, Santorum, announced he was suspending his campaign.
Gingrich and Paul have stubbornly refused to bow out of the race despite the widening gap between them and Romney. Gingrich still wasn't quite ready to call it quits when he talked to Fox News commentator Sean Hannity earlier Tuesday.
"We're going to be in Tampa [Fla.] whether it's as an active candidate or as a leader in the party," Gingrich said. "I've said all along, the morning [Romney] gets to 1,144 delegates then he's the nominee, but he's not there yet."
Delaware was the best chance the cash-strapped Gingrich had of picking up a win. He invested a lot of time campaigning to try to collect the state's 17 delegates.
Gingrich is trying to add another state to his win column after picking up only his home state of Georgia and South Carolina. He needs five wins to appear on the Republican National Convention ballot in Tampa.
Pennsylvania had 72 delegates at state, while New York had 95, Connecticut 28, Rhode Island 19 and Delaware 17.
One pollster had said Romney should be prepared for a sympathy vote in Pennsylvania for Santorum, who represented the Keystone State in the U.S. Senate from 1995 to 2007, the Albany, N.Y., Times Union reported. But Romney was taking more than 50 percent of the vote in Pennsylvania, early returns showed.