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

But Monday's events in Tampa, Florida have all been cancelled amid concern over the path of Tropical Storm Isaac. The storm will miss Tampa, but is set to hit the Gulf coast while the rejigged convention is in full swing.

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

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

But Monday's events will be limited to a symbolic opening and an immediate adjournment, with the evening agenda compressed on the following three nights.

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 virtually seven years to the day since Hurricane Katrina devastated New Orleans.