Republicans have bestowed their presidential nomination on Mitt Romney, turning to the former Massachusetts governor and multi-millionaire businessman to drive Barack Obama from the White House and usher in a new era of small-government conservatism.

The overwhelming, enthusiastic vote of delegates at the Republican National Convention in Tampa, Florida, belied Mr Romney's long, difficult road to the party's nomination: losing to Senator John McCain four years ago and fending off a series of rivals in a brutal nomination fight this year.

In the end, Republicans cast aside doubts about Mr Romney's conservative credentials and bet that American voters would be persuaded that his business acumen was just what the country needed in dreary economic times.

But Republican exuberance was tempered as Hurricane Isaac slammed into the southern Louisiana coast early today and headed towards New Orleans, striking the same region hit by Hurricane Katrina seven years ago.

The storm prompted Republicans to cancel the first day of the convention. Although it no longer threatens Tampa, Republicans wanted to avoid holding a boisterous political celebration just as the storm was unleashing its fury.

With Mr Romney's nomination now official, and Mr Obama's assured at next week's Democratic convention in Charlotte, North Carolina, US voters will face a clear-cut clash of ideologies.

Mr Romney, conservative on social issues such as gay marriage and abortion, favours cutting taxes, slashing the government and repealing Mr Obama's signature healthcare overhaul - even though it was modelled after one of his own programmes as governor.

Mr Obama is liberal on social issues, wants to raise taxes on the wealthiest Americans and sees government as a potential force for good.

Polls show the race a dead heat, with the economy the top issue in the campaign. Voters say they trust Mr Romney more on economic issues, but find Mr Obama to be the more likeable candidate.

Mr Romney is due to make his acceptance speech tomorrow night local time, though those plans could be affected by the weather.

In the state-by-state roll call of convention delegates, Mr Romney won 2,061 to 190 for Ron Paul, a libertarian-leaning Texas congressman. Mr Paul's supporters chanted and booed after the convention adopted rules they opposed, but were powerless to block, to prevent those votes from being officially registered. "Shame on you," some of his supporters shouted from the floor.

The highlights of the convention were the speech today by Romney's wife Ann and the keynote address by New Jersey's outspoken governor Chris Christie, a star of the party. Both spoke during prime US television time when all the major networks were airing the convention live.

Mrs Romney's speech was meant to cast her husband, lampooned by comedians as robotic and denounced by Democrats as lacking compassion, in a soft and likeable light. Mr Romney was in the hall for her speech.

She lovingly talked of her 43-year marriage, noting her own experiences battling multiple sclerosis and breast cancer. She described her husband as a man who wakes up every day determined to solve the problems that others say cannot be solved.

"This man will not fail," she said. "This man will not let us down."

Mr Romney appeared on stage briefly at the end of the speech, kissing his wife, to wild cheers.

Mr Christie's and Mrs Romney's speeches followed a long series of addresses by other top party officials, praising Mr Romney and blasting Mr Obama.

Reince Priebus, chairman of the Republican National Committee, said Mr Obama had "never run a company. He hasn't even run a garage sale or seen the inside of a lemonade stand".

Rick Santorum, one of Mr Romney's primary rivals, told the convention that under Mr Obama, the American dream of freedom and opportunity had become a "nightmare of dependency" with almost half of the country receiving some form of government benefit.

Also, before nominating Mr Romney, Republicans approved a party platform calling for cutting taxes as a way to stimulate the economy, ending abortion and repealing Mr Obama's healthcare plan.

Congressman Paul Ryan, Mr Romney's vice-presidential running mate, will speak today. Republican delegates officially nominated him for the second-in-command spot yesterday, shortly after they named Mr Romney.

While Republicans gathered in Florida, Mr Obama campaigned in Iowa and Colorado as he set out on a tour of college campuses in hopes of boosting voter registration among students, who tend to support him.

Before leaving the White House, he made a point of appearing before reporters to announce the government's latest steps to help those in the path of Hurricane Isaac.

He signed a declaration of emergency for Mississippi and ordered government aid to supplement state and local storm response efforts in the state.