Media playback is unsupported on your device Media caption Republican National Convention officially opens in Tampa, Florida

US Republicans have opened their national convention as they prepare to formally nominate Mitt Romney as Barack Obama's White House challenger.

But the convention in Tampa, Florida, was immediately adjourned until Tuesday amid concern over Tropical Storm Isaac.

The storm will miss Tampa, but is set to hit the Gulf coast while the rejigged convention is in full swing.

Concerns remain that the storm could overshadow key speeches by Mr Romney and running mate Paul Ryan.

The convention has been meticulously planned to highlight the personal story of Mr Romney, a former business star and governor of Massachusetts.

'A new low'

But Monday's events were limited to a symbolic opening and an immediate adjournment by Republican National Committee chairman Reince Priebus, with the evening agenda compressed on the following three nights.

Key convention speeches Tuesday: Ann Romney, New Jersey Governor Chris Christie Wednesday: Former Secretary of State Condoleezza Rice, vice-presidential candidate Paul Ryan Thursday: Florida Senator Marco Rubio, Mitt Romney

Mr Ryan, a Wisconsin congressman known for his work on Republican budget plans, will face by far his biggest national exposure when he takes to the stage on Wednesday night.

But he could make his speech against a backdrop of images of high winds and storm surges lashing the Gulf coast, almost seven years to the day since Hurricane Katrina devastated New Orleans.

Ahead of his appearance at the convention, Mr Ryan said on Monday: "We're not just picking the next president for a few years. We are picking the pathway for America for a generation.

"And what Mitt Romney and I pledge is to make sure that you get to choose, what kind of country do we want to have? What kind of people do we want to be?"

During the convention, Republicans are expected to focus on their economic message of tax and entitlement reform, and to depict Mr Obama's policies as the reason for a slow recovery.

As convention week began, a Washington Post/ABC News poll suggested the two candidates were essentially tied among registered voters, with 50% of all surveyed saying they had "enough" information to make their choice.

Over the weekend, Mr Romney accused Mr Obama of running a campaign built on "anger and divisiveness".

Mr Romney told Fox News the Democratic campaign had hit a "new low" by trying to link him to comments by Todd Akin, a Senate candidate for Missouri, who was roundly criticised for his comments on rape, pregnancy and abortion.

Mr Akin said that women who were raped rarely became pregnant, but later apologised for using his words "in the wrong way".

Language promoting a constitutional ban on abortion with no exceptions for rape or incest has been included in the Republican platform and will be voted on at the convention.

'Images of revelry'

While postponing many of Monday's events because of the storm will shorten some of the convention's speeches, party officials are planning to continue as normal.

The soon-to-be officially nominated Republican candidate has been quite insistent in recent interviews: he is like Popeye

"Our thoughts are with the people that are in the storm's path and hope that they're spared any major destruction," Mr Romney said when asked on Monday if he had any words for the people of New Orleans.

Tropical Storm Isaac is forecast to make landfall on Tuesday or Wednesday as a hurricane, possibly near New Orleans.

The city was devastated by Hurricane Katrina seven years ago, and Wednesday would be the anniversary of that storm's landfall on the Gulf coast.

"Images of revelry by Republicans at a time of suffering by other Americans - no party wants those optics," Steve Schmidt, who led the Republican presidential effort of John McCain in 2008, told the New York Times ahead of the convention.

Louisiana Governor Bobby Jindal, who was scheduled to speak at the convention, cancelled his trip, as did his Alabama and Mississippi counterparts.

In other developments, a former Republican governor of Florida, Charlie Crist, is scheduled to speak at the Democratic national convention next week.

Mr Crist, who ran as an independent after losing the Florida Republican primary for Senate, endorsed Mr Obama in an editorial on Sunday, explaining the Republican party had "pitched so far to the extreme right".