The battle for the 2012 Republican presidential nomination heats up on Thursday with the entry of default front-runner Mitt Romney into a race devoid of a clear favorite.In prepared remarks released ahead of announcing his candidacy, the ex-governor of Massachusetts said that President Barack Obama "has failed America," and vowed to increase the country's job growth.Considered next-in-line after losing the 2008 nomination to Senator John McCain, strong name recognition has helped Romney, 64, top many early opinion polls, albeit by slim margins in a tightly-bunched field.