The presumptive Republican presidential nominee swept primaries in New York, Pennsylvania, Connecticut, Rhode Island and Delaware Tuesday, topping 820 delegates as he moved closer to 1,144 needed to formally secure the nomination.
"Americans have always been eternal optimists," Romney told supporters in Manchester, N.H. "Over the last 3 1/2 years, we have seen hopes and dreams diminished by false promises and weak leadership. Everywhere I go, Americans are tired of being tired.
"To all of the thousands of good and decent Americans I've met who want nothing more than a better chance, a fighting chance, to all of you I have a simple message," Romney said. "Hold on a little longer. A better America begins tonight."
He argued that President Barack Obama failed in office.
"Because he has failed, he will run a campaign of diversions and distractions and distortions," Romney said. "That kind of campaign may have worked at another place and in a different time -- but not here and not now."
Then, in a twist on Bill Clinton's 1992 campaign message against incumbent Republican George H.W. Bush, he added, "It's still about the economy, and we're not stupid."
Clinton's phrase "It's the economy, stupid" referred to the notion that he was a better choice than Bush because Bush had not adequately addressed the economy, which had recently undergone a recession.
Romney described Obama as a president who arrived in office to great fanfare, only to fall short.
"The last few years have been the best that Barack Obama can do, but it's not the best America can do," he said.
Former U.S. Sen. Rick Santorum of Pennsylvania, who left the race two weeks ago, told CNN Tuesday night he would support Romney as the nominee, claiming that despite their differences during the nomination battle, the former governor was far superior to Obama.
Santorum planned to meet with the former Massachusetts governor May 4, chief Santorum strategist John Brabender told The Wall Street Journal.
Former U.S. House Speaker Newt Gingrich of Georgia didn't quit the race after losing in Delaware, a primary he considered crucial, but he told supporters, "Over the next few days, we're going to look realistically at where we're at."
Rep. Ron Paul of Texas said he would continue campaigning, even though he is at the back of the GOP pack.