Mitt Romney has declared victory in Nevada's Republican caucuses, cementing his frontrunner status in the race to be the party's presidential candidate.With just over 70% of the votes counted, he had secured 47.6% of the vote - well ahead of his nearest rival Newt Gingrich on 22.6%.It is Mr Romney's second victory in a week, following his success in Florida.Mr Gingrich - a former house speaker - has vowed to fight on until the party's convention in August.