Obama declares state of emergency in Louisiana

Tropical Storm Isaac is heading for New Orleans, possibly as early as Tuesday night, nearly seven years to the day after Hurricane Katrina devastated the city.

In the meantime, the Republican national convention has begun as Tropical Storm Isaac reduced the gala opening of Mitt Romney's party coronation to a symbolic session of less than two minutes.

The US national weather service said late on Monday that Isaac was expected to become a hurricane "soon". At 10:00 local time, the storm was about 430km from New Orleans, moving towards the city at a speed of 17km per hour.

The storm was exhibiting sustained winds of 110km per hour, and was expected to make landfall sometime between Tuesday afternoon and night.

The weather service said, however, that it was not expected to be as strong a storm as Hurricane Katrina, which battered the city in 2005.

Forecasts put Isaac on a direct path for New Orleans, seven years after Hurricane Katrina devastated the city and killed 1,800 people in the country's worst natural disaster in living memory.

US President Obama approved Louisiana's request for a federal disaster declaration, making available federal funds for recovery activities such as clearing debris.

Earlier, the governors of Louisiana, Florida, Mississippi and Alabama declared emergencies in their states.

The Republican governors of Alabama, Louisiana and Mississippi have cancelled their trips to their party's convention to focus on disaster prevention efforts.

Isaac is already a large storm and could bring significant damage to areas within hundreds of miles of its centre, officials warn.

Isaac has so far killed at least 24 people in Haiti and the Dominican Republic. The storm wrought significant flooding and damage in the Caribbean.

The original script had seen Romney formally nominated to take on Barack Obama, the US president, in the November 6 election by thousands of delegates, launching a succession of well-honed speeches by leading party figures.

That roll call will now take place on Tuesday afternoon as the convention in Tampa, Florida gets under way for real.

Lying neck-and-neck with Obama in the polls 10 weeks before election day, Romney hopes to use the convention to recast his image after months of damaging White House attacks on his tax secrecy and business record.

Many US voters do not tune in until the convention season starts - Obama and the Democrats hold their convention next week in Charlotte, North Carolina, - so Tampa provides Romney with a golden opportunity to reset the narrative.

The run-up to the convention was marred by incendiary remarks from Todd Akin, a Republican congressman seeking a Senate seat in Missouri who suggested women's bodies spontaneously prevent pregnancy after a "legitimate rape".

The Romney camp is thus anxious to get the campaign back on message, billing the former Massachusetts governor as a successful businessman with the acumen to turn around the flagging US economy and get the country back on track.

A convention speech on Tuesday by Romney's wife Ann will highlight his human side, while former Olympians take to the floor on Thursday to remind Americans that he saved the 2002 Salt Lake City Olympics from bankruptcy.

But the storm presents the candidate with the spectre of a natural disaster casting a pall over his big moment, and he must tread a fine line between driving his political ambitions and showing sensitivity to those in harm's way.

National polls have shown a neck-and-neck race between Romney and Obama for weeks. A Washington Post and ABC News poll out Monday showed the challenger one point ahead, 47 per cent to 46 per cent.

Other surveys show Obama with crucial leads in some of the most important swing states that could decide the election.