To roaring cheers at the Republican National Convention in the packed Tampa Bay Times Forum, the delegation from New Jersey put Romney above the 1,144-delegate threshold, ensuring he will be the GOP challenger to President Barack Obama in November.
Earlier, the 2,200-plus convention delegates approved a conservative platform that called for less government, opposed same-sex marriage and endorsed a "human life amendment" to ban abortion with no specific exceptions for cases of rape, incest or when the mother's life is threatened.
Romney and his running mate, Rep. Paul Ryan of Wisconsin, will be formally nominated on Thursday, and Romney's acceptance speech that night will conclude the convention that had its agenda delayed by Hurricane Isaac, which hit Louisiana as early evening speakers addressed the delegates.
Republican officials appeared determined to stick to a tightened three-day schedule that kicked into full gear earlier in the day with official business and speeches accusing Obama of failed leadership and undermining the American dream.
Republican National Committee Chairman Reince Priebus launched the litany of attacks, saying that another term for Obama and Vice President Joe Biden will mean "four more years of failure."
In reference to Obama, Priebus said "he hasn't even run a garage sale or seen the inside of a lemonade stand." The nation needs a president "with real experience in a real economy," Priebus added. "Mitt Romney will be that president."
Other speakers continued the effort by Priebus to frame the election as a referendum on the policies of Obama.
"The American people are still asking 'where are the jobs,' but President Obama only offers excuses instead of answers," House Speaker John Boehner told the delegates. "His record is a shadow of his rhetoric. Yet he has the nerve to say that he's moving us forward, and the audacity to hope that we'll believe him."
Boehner also said "we can do better," adding that "it starts with throwing out the politician who doesn't get it, and electing a new president who does."
"President Romney will keep his word and his courage, too," Boehner said. "He'll keep faith with the idea that government exists to serve the people, and the people build the economy."
Rep. Dan Benishek of Michigan complained earlier that Democrats in Washington were "spending away our children's and grandchildren's futures," while Rep. Tim Scott of South Carolina serenaded Obama with a chorus of the Ray Charles tune, "Hit the Road, Jack."
At a campaign event in Iowa, Obama said Tuesday that he expected the GOP convention to be "a pretty entertaining show."
"I am sure they will have some wonderful things to say about me," the president said. "But what you won't hear from them is a path forward that meets the challenges of our time."
Romney and his wife, Ann, flew into Tampa on Tuesday, a few hours before Ryan and his family arrived. Ann Romney is one of the featured speakers Tuesday night, along with the keynote address to be given by New Jersey Gov. Chris Christie. Ryan will speak to the convention Wednesday night.
In a sign of lingering internal division in the party after a rugged primary campaign won by Romney, rival candidate Rep. Ron Paul of Texas was thronged by supporters when he entered the convention floor before Tuesday's session convened. Some Paul supporters shouted "let him speak," referring to their candidate's exclusion from the convention agenda.
Paul supporters later protested a rule change adopted by the convention that they believe will hinder their kind of grassroots campaign in the future. They also cheered wildly when Paul received any delegates in the state-by-state roll call.
Convention organizers acknowledged they were monitoring Isaac, which made landfall one day before the seventh anniversary of Hurricane Katrina's devastating march through Louisiana and Mississippi.
They are concerned about the perception of a celebratory convention with colorful balloons and soaring rhetoric juxtaposed with a storm slamming the Gulf Coast at the same time, potentially evoking memories of Katrina's wrath and ensuing criticism of the response of the Republican administration of George W. Bush.
However, Romney and Republicans are reluctant to lose any more of their best opportunity to define the candidate for the American people with less than three months until the election.
"He is a very shy guy. He is a humble guy. He doesn't like to talk about himself. That's who he is," Boehner said of Romney in an interview Tuesday with CNN. "But I've known Mitt Romney for a long time. Decent, honest, hardworking guy. And I think Thursday he'll have a chance to reintroduce himself to the American people."
The acceptance speech will give Romney a major chance to speak for himself, rather than be defined by either his Republican primary rivals or by Obama, Boehner said.
"He's been locked in this Republican primary and then locked into this battle with the president," the Ohio Republican added. "And as a result, people have all different kind of views of him. So I think Thursday night is clearly an important speech for him and I think he'll have a chance to reintroduce himself to the American people, most of whom are just paying attention now."
For Romney, 65, the nomination will put him closer to the goal he first sought in 2007 by running for president after serving as a Republican governor for four years in traditionally Democratic Massachusetts.
The multimillionaire businessman lost to veteran Sen. John McCain in the Republican primary campaign in 2008, then spent the next two years preparing for another try. He emerged victorious from the primaries during which rivals challenged his conservative credentials, and continues to walk a political tightrope that attempts to galvanize right-wing support while also trying to appeal to moderates and independent voters.
The Romney campaign and Republicans have a big fund-raising advantage over Obama and Democrats, due in part to unlimited private donations to conservative super-PACs.
In a new ad Monday, the liberal MoveOn.org's political wing attacked Romney for what the group says is his focus on the rich. The 30-second spot, titled "Stepping on the Middle Class," features Romney and Ryan lookalikes stepping on people dressed as typical middle-class Americans -- firefighters, students, seniors and children -- as they walk across the convention floor to accept the GOP nomination.
The latest CNN/ORC International poll indicates a dead heat between Romney and Obama, with new numbers released Sunday showing that 53% of likely voters believe Obama is more in touch with their needs, compared with 39% for Romney.
Obama leads by an equal margin when it comes to being in touch with the middle class, and six in 10 say Obama is in touch with the problems facing women today, with just over three in 10 feeling the same way about Romney.
Romney leads 48% to 44% over Obama on managing the government effectively and has a six-point advantage on having a clear plan for fixing the nation's problems. Both figures are within the survey's margin of error.
"The challenge facing Romney at the GOP convention is to build on those managerial strengths while at the same trying to convince average Americans that he is in touch with their problems. Obama's personal characteristics, for the moment, outshine Romney's," CNN Polling Director Keating Holland said.
On specific issues, the poll results show a similar dynamic: Obama is generally ahead on foreign policy and social issues, while Romney is generally preferred on economic issues.
Another new poll released Monday showed Romney and Obama neck-and-neck in Florida, a battleground state, and North Carolina, where the Democratic convention will take place next week.
Republicans say the Republican convention must focus on Romney's character and show how he can lead the nation to economic prosperity, which is the top issue with voters.
Ann Romney's speech Tuesday night was expected to address concerns about her husband's perceived stiffness by presenting him in a more personal, softer light.
In excerpts of the speech released by convention organizers, Mrs. Romney tells of challenges the couple faced, including her health problems with multiple sclerosis and breast cancer. She describes Romney as a dependable, loving husband and father, according to the excerpts.
"I can't tell you what will happen over the next four years," Mrs. Romney will say. "But I can only stand here tonight, as a wife, a mother, a grandmother, an American, and make you this solemn commitment: This man will not fail. This man will not let us down. This man will lift up America."
The Obama campaign, anticipating a Romney branding effort by Republicans, released a movie trailer-style video Sunday that previewed a "do-over" moment for Romney.
In a statement accompanying the video, the Obama campaign said it is "presenting Americans with an epic cinematic preview of Mitt Romney's 'convention reinvention' -- the Do-Over moment that voters have grown to expect -- because they've seen this movie before."
The GOP platform contains traditional conservative planks, including support for the "human life amendment" with no exceptions to an abortion ban. That runs counter to overall public opinion in America, especially among women, a demographic that polls show favoring Obama far more than Romney at this point.
Romney and Ryan, the conservative House Budget Committee chairman, have been asked repeatedly about differences between their personal views on whether there should be limited abortion exceptions.
Romney's Mormon faith calls for the narrow exceptions, while Ryan, a devout Catholic, supports a blanket ban. The campaign has made clear the ticket supports Romney's stance, which contrasts with the party platform.
CNN's Kevin Bohn, Paul Steinhauser, Dana Davidsen, Ashley Killough, Dana Bash, Deirdre Walsh, Halimah Abdullah, Martina Stewart, Shawna Shepherd, Rachel Streitfeld and Mark Preston contributed to this report.