Mitt Romney vowed to rescue the US economy and create jobs as he accepted the Republican presidential nomination at the climax of a convention that sought to humanise the candidate.

The former Massachusetts governor told Americans that Barack Obama had singularly failed to deliver the "hope and change" he had promised and that the country must elect him to save an economy crippled by wrong-headed policies.

Romney's elevation to official challenger to Obama in the November election comes more than five long years after he launched his first White House bid and with the current race neck-and-neck and dependent on a handful of key states.