Media playback is unsupported on your device Media caption Republicans in Florida say that it is time for change in the US

US Republicans have begun the first full day of their re-organised national convention, after a one-day delay because of Hurricane Isaac.

Amid pageantry in Tampa, delegates formally nominated Mitt Romney as Barack Obama's White House rival.

Later, there will be speeches by Ann Romney, South Carolina Governor Nikki Haley and a keynote address from New Jersey Governor Chris Christie.

Concerns remain over Isaac, which is nearing New Orleans and the Gulf Coast.

The storm has strengthened into a hurricane, and appears set to affect a stretch of coast several hundred miles wide between Louisiana and Florida.

It comes almost seven years to the day since Hurricane Katrina devastated New Orleans.

'Turning it around'

Tuesday's proceedings began at 14:00 EST (18:00 GMT) with the singing of the US national anthem and the first addresses.

After the roll-call of party delegates during the afternoon, the official nomination of the presidential and vice-presidential candidates Mitt Romney and Paul Ryan took place.

Key convention speeches Tuesday: Ann Romney, House Speaker John Boehner, South Carolina Governor Nikki Haley, former US senator Rick Santorum, New Jersey Governor Chris Christie Wednesday: New Mexico Governor Susana Martinez, Arizona Senator John McCain, former Secretary of State Condoleezza Rice, vice-presidential candidate Paul Ryan Thursday: Former House Speaker Newt Gingrich, former Florida Governor Jeb Bush, Florida Senator Marco Rubio, presidential candidate Mitt Romney In pictures: Republican convention

After a recess, the first of three days of speeches are to begin in the evening.

Mr Romney's wife, Ann, is expected to seek to "humanise" her husband, a former business star who remains something of an unknown quantity to many voters.

She told reporters her husband would attend her speech, adding: "It's going to be fun for him to be there."

Other highlights will include a slew of recently elected governors including Wisconsin's union-busting Scott Walker, South Carolina's rising star Nikki Haley, and New Jersey's Chris Christie, who was touted as a potential vice-presidential pick.

Gov Christie told reporters that for those Americans who are not yet sold on Mr Romney, "you start turning it around tonight".

Mr Romney, a former Massachusetts governor, travelled to Tampa on Tuesday.

Isaac's path past Tampa meant Monday's events were limited to a symbolic opening and an immediate adjournment, with the evening agenda compressed into the following three nights.

Media playback is unsupported on your device Media caption Can Mitt Romney close gender gap?

Several Republican governors have stayed away from the convention because of the storm, including Louisiana's Bobby Jindal, who was originally scheduled to speak.

A carefully chosen line-up of Republican speakers from across the country will continue on Wednesday and Thursday, with each evening capped by the two nominees - Mr Romney and vice-presidential running mate Paul Ryan.

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.

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."

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

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

As convention week began, a Washington Post/ABC News poll suggested the two candidates were essentially tied among registered voters.

Correspondents say the presidential race will ultimately be decided in a handful of battleground states - including Florida, Ohio, Virginia, Michigan and Nevada - where opinion polls show the race is tightest.

A candidate must amass at least 270 electoral college votes out of a total of 538 in order to win, and the votes are allocated according to population size.

Many opinion polls also suggest that Mr Romney trails Mr Obama in "likeability" ratings, something analysts say the Romney campaign is hoping to boost over the course of the convention.

Mr Obama was still campaigning on Tuesday, with trips planned at college towns in Iowa and Colorado.

The Democrats will hold their own convention in Charlotte, North Carolina, next week.