Republican presidential candidate Mitt Romney's choice of running mate could shift the race for the White House into a debate between two sharply contrasting views of government spending and debt.

In choosing Wisconsin Congressman Paul Ryan – a budget hawk with a provocative plan to cut government spending – Mr Romney has taken the biggest gamble of his candidacy.

He has embarked on a high-risk, high-reward strategy of aligning himself with Mr Ryan, whose budget plan would cut taxes and restructure safety-net social programmes including Medicare, the popular government-backed health insurance scheme.

Democrats say Mr Ryan's plan would amount to draconian cuts in programmes that help protect the nation's most vulnerable people, including the poor and the elderly. The potency of such an argument, particularly in crucial states such as Florida, which has a large elderly population, was clear on Saturday.

Shortly after he announced Mr Ryan as his running mate, Mr Romney's campaign emphasised to reporters that the selection did not mean Mr Romney supported Mr Ryan's entire budget plan.

Mr Romney needs a burst of energy for his campaign after falling behind Democratic President Barack Obama in recent polls.