After returning from a tour of relief preparations in Mississippi, Mr McCain said convention delegates needed to "take off our Republican hats, and put on our American hats and we say America, we're with you".
The Arizona senator's campaign chartered a jet to fly worried delegates back to their home states threatened by the hurricane.
The BBC's Adam Brookes, in Minnesota for the convention, says the Republicans are keen to avoid the kind of political damage incurred by the Bush administration's clumsy response to Hurricane Katrina three years ago.
The party clearly cannot afford to be seen hosting glamorous political events, while the people of New Orleans are once again fleeing their city, he says.
In 2005, three-quarters of New Orleans was flooded by a storm surge that claimed more than 1,800 lives in coastal areas.
All eyes will now be on the city's levees, which Category Three storm Katrina swept away under a wall of mud and water.
Plainly the backdrop of images of destruction reminding Americans of Katrina will be horrible for the Republicans
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