The hall erupted in cheers when Romney strolled on stage and shared a hug and kiss with his wife of more than 40 years.
"This man will not fail. This man will not let us down," Mrs. Romney said in a prime-time speech that sounded at times like a heart-to-heart talk among women and at times like a testimonial to her husband's little-known softer side.
"It's the moms who always have to work harder, to make everything right," she said. And she vouched firmly for her husband, who lags behind Obama in surveys among women voters: "You can trust Mitt. He loves America."
Earlier, the Romneys watched on television at a hotel suite across the street from the convention hall as delegates sealed his hard-won victories in the primaries and caucuses of last winter. They ended the evening together in a VIP box just above the convention floor.
To send Romney and ticketmate Paul Ryan into the fall campaign, the convention quickly approved a conservative platform that calls for tax cuts - not government spending - to stimulate the economy at a time of sluggish growth and 8.3 percent unemployment.
Republican mockery of Obama began almost instantly from the podium at a convention postponed once and dogged still by Hurricane Isaac. The Democratic president has "never run a company. He hasn't even run a garage sale or seen the inside of a lemonade stand," declared Reince Priebus, chairman of the Republican Party.
More than eight hours in length, the session inside the Republicans' red-white-and-blue-themed convention hall passed up no opportunity to broaden Romney's appeal. Speakers included Hispanic candidates for office; former Rep. Artur Davis, a one-time Democrat and member of the Congressional Black Caucus; businessmen and women and former Sen. Rick Santorum of Pennsylvania, Romney's most persistent, conservative nemesis in the nominating campaign.
"Leadership matters," declared New Jersey Gov. Chris Christie, keynote speaker and not coincidentally a Republican from a majority-Democratic state. "It's time to end this era of absentee leadership in the Oval Office and send real leaders to the White House."
Mrs. Romney's appearance was the highlight of the night, and it turned the proceedings into something of a his-and-hers convention.
"I read somewhere that Mitt and I have a "storybook marriage. Well, in the storybooks I read, there were never long, long, rainy winter afternoons in a house with five boys screaming at once," she said.
"A storybook marriage? No, not at all. What Mitt Romney and I have is a real marriage," she added in an appearance meant to cast her multimillionaire-businessman-turned politician in a softer, more likable light.
While there was no doubt about Romney's command over the convention, the residue of a heated campaign for the nomination was evident inside the hall.
Texas Rep. Ron Paul, who never won a primary or caucus, drew several dozen delegate votes. Earlier, his supporters chanted and booed after the convention adopted rules they opposed, but were powerless to block, to prevent those votes from being officially registered.
Opinion polls made the race a close one as the Republicans' days of pageantry and speechmaking began in earnest, and the man tapped to deliver the keynote address set the stakes.
"Conventions are always huge for a challenger, because they're the ones introducing themselves" to the voters, said New Jersey Gov. Chris Christie.
Convention planners squeezed two days of speeches and other convention business into one after scrapping Monday's scheduled opener because of fears that Isaac would make a direct hit on the Florida Gulf Coast.
That threat fizzled, but it was instantly replaced by another - that Republicans would wind up holding a political celebration at the same time the storm turned its fury on New Orleans, devastated almost exactly seven years ago by Hurricane Katrina.
Romney's convention planners said they were in frequent contact with weather forecasters, but they declined to discuss what contingency plans, if any, they had to accelerate plans for him to deliver a formal acceptance speech Thursday night.
"This is obviously the biggest speech of my life," Mrs. Romney said as she visited the custom-made podium to prepare for her remarks.
Ratification of a party platform was prelude to Romney's nomination, a document more conservative on abortion than the candidate.
On economic matters, it backs extension of the tax cuts enacted in 2001 and 2003 and due to expire at year's end, without exception. It also calls for an additional 20 percent reduction in income tax brackets that Romney favors.
In a time of 8.3 percent unemployment and the slowest economic recovery in the post-World War II era, that went to the crux of the campaign for the White House.
By contrast, Obama wants to allow existing tax cuts to expire on upper income taxpayers, and has criticized Romney's overall economic plans as a boon to millionaires that would raise taxes on the middle class.
The GOP platform also pledges that a Republican-controlled Congress will repeal, and Romney will sign, legislation to repeal the health care legislation Obama won from a Democratic-controlled Congress. So, too, for the measure passed to regulate Wall Street in the wake of the 2008 economic collapse.
On abortion, the platform says, "The unborn child has a fundamental individual right to life which cannot be infringed."
Romney opposes abortions, except in cases of rape, incest, or when "the health and life of the mother" are at stake, he said in a convention week interview.
Obama, who accepts renomination at the Democratic National Convention in Charlotte, N.C., next week, campaigned in Iowa Tuesday as he set out on a tour of college campuses in battleground states in hopes of boosting voter registration among college students.
Before departing the White House, he made a point of appearing before reporters to announce the government's latest steps to help those in the way of Isaac. He signed a declaration of emergency for Mississippi and ordered federal aid to supplement state and local storm response efforts in the state.
His surrogates did their best to counter Romney and the Republicans.
Los Angeles Mayor Antonio Villaraigosa, dismissing GOP attempts to woo Hispanic voters, said, "You can't just trot out a brown face or a Spanish surname and expect people are going to vote for your party or your candidate." He added, "This is a party with a platform that calls for the self-deportation of 11 million people."
Hispanics strongly favor Obama, according to public polls, and Romney and his party have been seeking to win a bigger share of their votes by emphasizing proposals to fix the economy rather than ease their positions on immigration.
Female voters, too, prefer the president over his challenger, and Democrats have done their best to emphasize GOP opposition to abortion and even suggest the party might try and curtail access to contraceptives if it wins power.
Whatever the impact of those issues, the polls show the economy is overwhelmingly the dominant issue in the race, and on that, the voters narrowly say they trust Romney more.
In an AP-GfK poll taken Aug. 16-20, some 48 percent of registered voters said they trust Romney more on economic issues, to 44 percent for Obama.
However, a Washington Post-ABC News in the days immediately before the convention found that 61 percent of registered voters said Obama was more likable, and 27 percent said Romney.
The convention took place in an atmosphere of security that was both stringent and selective. Thousands of police from all over the country, joined by National Guard troops, Secret Service and others, stood in small groups at checkpoints, demanding those entering a secure area display proper credentials numerous times.
But former Michigan Gov. John Engler and an aide were hustled to the front of a long line waiting to clear security at one building.
Aside from Paul, Romney's long-ago rivals for the party nomination had bit roles at his convention, if that.
Michele Bachmann and Herman Cain posed for a photo after running into each other at the convention center. Newt Gingrich and Rick Santorum were also in town, as well, both with speaking slots, unlike Bachmann and Cain.
Associated Press writers Brian Bakst, Thomas Beaumont, Tamara Lush, Brendan Farrington, Julie Mazziotta, Steve Peoples, Kasie Hunt and Philip Elliott in Florida and Steven Ohlemacher, Alicia A. Caldwell and Jennifer Agiesta in Washington contributed to this report.