After struggling for months to prevail over unexpectedly persistent rivals, the Republican nominee-in waiting was eager to turn the political page and launch the campaign against Obama whom he accused of "false promises and weak leadership."
The former Massachusetts governor spoke as he pocketed primary victories in Connecticut, Rhode Island, Delaware, Pennsylvania and New York in the first contests since his chief rival, former Pennsylvania Sen. Rick Santorum, conceded the nomination. 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.
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.
In an indication that Romney was treating the moment as something of an opening of the general election campaign, his speech seemed aimed at the millions of voters - non-conservatives and others - who have yet to pay close attention to the race for the White House. He urged all those struggling to "hold on a little longer; a better America begins tonight."
Romney blended biographical details, an attack on Obama and the promise of a better future, leaving behind his struggle to reassure conservative voters who have been reluctant to swing behind his candidacy.
"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," he said. "This does not have to be. It is the result of failed leadership and of a faulty vision."
Romney spoke dismissively of the president's tenure in office. "Government is at the center of his vision. It dispenses the benefits, borrows what it cannot take and consumes a greater and greater share of the economy," he said.
He added that if the president's hard-won health care reform law is fully installed, "government will continue to control half the economy, and we will have effectively ceased to be a free enterprise society."
By contrast, he said, "I see an America with a growing middle class, with rising standards of living. I see children even more successful than their parents..."
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.
The nominating campaign still has some loose ends, including the pursuit of delegates to the Republican National Convention in Tampa, Florida, in late August.
Romney is still more than 400 Republican National Convention delegates shy of a nominating majority, although he is far ahead of his most persistent rivals. There were 209 delegates at stake in Tuesday's primaries.
Romney began the day with 698 delegates of the 1,144 needed for the nomination, compared with 260 for Santorum, 137 for Newt Gingrich, the former speaker of the House of Representatives, and 75 for Texas Rep. Ron Paul.
Santorum suspended his campaign two weeks ago rather than risk losing a primary in his home state of Pennsylvania.
Gingrich, too, seemed to be heading toward the sidelines, but first he wanted to see the outcome of the primary in Delaware, where he has campaigned in recent days and has pocketed a few endorsements. Jackie Cushman Gingrich, his daughter, said the former House speaker intended to reassess his debt-strapped candidacy on Wednesday.
The nomination in hand, Romney has begun focusing more on Obama in recent days, campaigning in key battleground states, appointing an aide to oversee his search for a vice presidential running mate and accelerating his fundraising for the fall.
On Monday, he offered support for Obama's call for legislation to prevent an increase in the interest rate on some student loans. In a second move toward the middle, he said his campaign was reviewing legislation to let young illegal immigrants brought to the U.S. by their parents apply for non-immigrant visas.
Under a measure being drafted by Florida Sen. Marco Rubio, a potential vice presidential pick for Romney, the immigrants affected by the legislation would be allowed to study or work in the United States but would not have a special path to citizenship.