Brian Snyder / REUTERS

Supporters cheer as they wait for a speech by Mitt Romney in Manchester, N.H. on April 24, 2012.

Updated 9:48 p.m. ET - Mitt Romney declared victory in his quest to become the Republican presidential nominee on Tuesday and kicked off his general election campaign against President Barack Obama in earnest following a clean sweep of primaries in the Northeast.

Romney's performance in Connecticut, Delaware, New York, Pennsylvania, and Rhode Island allowed him to cap a tumultuous GOP primary cycle that extended longer than many expected. Romney's march toward the 1,144 delegates needed to secure the nomination appeared, at this point, to be all but a formality.

And, eager to begin prosecuting his case against Obama, Romney took a victory lap in the general election swing state of New Hampshire -- rather than appearing in any of the states hosting nominating contests tonight or in the future -- to declare, "a better America begins tonight."

"Tonight I can say thank you, America," Romney told a cheering crowd in the Granite State. "After 43 primaries and caucuses, many long days and more than a few long nights, 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 faced only token opposition from former House Speaker Newt Gingrich and Texas Rep. Ron Paul in Tuesday's contests. The former Massachusetts governor had all but assumed the status of presumptive Republican nominee two weeks ago, when his principal conservative rival, former Pennsylvania Sen. Rick Santorum, suspended his campaign.

Major Republican figures had finally begun to rally around Romney and offer their endorsements since that point, but he must still work toward winning the 1,144 delegates needed to formally secure the nomination. Romney had entered Tuesday having secured 698 of the necessary delegates, according to Associated Press projections, putting him on pace toward crossing the threshold in late May or early June.

Tonight's primaries may also signal the last gasp for Gingrich, as well, who had pinned his hopes of continuing his campaign on winning Delaware. But the ex-speaker offered no hint as to his future plans in brief remarks.

"I think it's a very substantial mistake for Gov. Romney to give a general election speech tonight in New Hampshire," Gingrich told reporters Tuesday. "He is not the nominee. I think it's a little insulting to people in these states." But he did indicate at a North Carolina event that "over the next few days, we're going to look realistically at where we're at."

Romney spent the evening focusing not on his remaining primary challenges, and instead trained his sights instead on the task of unseating Obama this fall.

"This has already been a long campaign, but many Americans are just now beginning to focus on the choice before the country. In the days ahead, I look forward to spending time with many of you personally. I want to hear what’s on your mind, hear about your concerns, and learn about your families," he said, promising to tell voters more about himself.

For Romney, that re-introduction is a delicate and important task. The most recent NBC News/Wall Street Journal poll found that more Americans -- 36 percent -- had a negative impression of Romney than the 33 percent who said they viewed the former Massachusetts governor positively.

On tests of whether he or Obama is seen as more easygoing and likable, or more in touch with the middle class, the president badly outpaces Romney.

Mindful of that, Romney kept his speech keyed in closely on pocketbook issues, warning of "diversions and distractions" from the central issue of the economy.

"It's still about the economy, and we're not stupid," Romney said, referencing the famous political maxim first employed by Bill Clinton's presidential campaign in 1992.

Obama's re-election team has been eager to reuse fodder from the primary season against the former Massachusetts governor in the context of the general election, underscoring the urgency for Romney to put the GOP contest to bed.

"Mitt Romney has spent the past year out on the campaign trail tearing down the president with a negative message that even Republicans who have endorsed him have criticized," said Ben LaBolt, Obama's campaign spokesman. "This marks the end of that monologue. Now he must put his record and his agenda next to the president’s."

Although Santorum dropped out two weeks ago, he’s among the conservatives who are yet to have thrown their support to Romney. NBC News learned Tuesday that the two men will meet on May 4 at a to-be-determined destination, though the meeting wasn't expected to produce an immediate endorsement.

That Romney had not yet won an endorsement before the primary in the state that Santorum had represented in Congress suggests that the rift between conservatives and the presumptive nominee has not yet fully healed.

Gingrich's persistence poses a minor challenge to that effort to unify the party, though the former speaker hints that he may soon address his future as a candidate.

Paul is also promising to forge ahead with his own campaign, perhaps through the late May primary in his native Texas. But Romney might have won the delegates he needs by that point.

NBC's Andrew Rafferty contributed to this report.

DOVER, DE -- Given the opportunity to work in Mitt Romney’s cabinet, Newt Gingrich says he would pass.

Asked by a radio host inside a small diner whether he would work for a Romney administration if given the opportunity, Gingrich answered “probably not” but “not because I am opposed to Mitt.”

“Look, if the choice does end up being Romney versus Obama, I can be very, very enthusiastic for Romney, that is a huge choice. But I had a very good life doing a lot of fun things,” the former House speaker said. “I am very happy to be an adviser. I did a lot of that in the Bush administration both on health care and national security.”

Romney seems to have all but sealed the nomination – he still has not reached the 1,144 delegate count threshold – but Gingrich says he has more explaining to do.

“I’m making the case for a very ideas-oriented, solutions-oriented kind of conservatism, and I think Romney is going to have to explain a lot more of what he would actually do as president,” he said.

Gingrich also leveled sharp charges toward President Obama as well at the Hollywood Diner, where he was interviewed on Elliott in the Morning radio show.

“The thing that Obama does that’s fascinating is,” Gingrich said. “On one hand he has all the instincts of a Chicago machine politician. On the other hand he has all the fantasies of a good college faculty member.”

And the media was mixed in as well – an institution Gingrich claims is Obama’s “enabler.”

“The elite news media gets up every morning thinking how do we protect this guy because he’s so wonderful that even if he’s totally destructive he’s destructive in such a fun way that we really like keeping here there, and how do we protect him from himself, because after all it’s not his fault that he’s crippling the economy, bankrupting the nation, and doing a series of weird things, because we know he really means well,” he said.

Gingrich will continue to campaigning heavily in both Delaware, where voters take to the polls on April 24, and North Carolina, votes on May 8, because he believes wins there may get him back in the game – keeping alive the dream of debating Obama one-on-one this fall.