In a detailed and passionate endorsement of his wife Hillary's former rival for the presidency, Clinton amplified the central argument of Obama's campaign - that voters face a choice between Democratic policies that lead to broad prosperity or Republican policies that benefit the wealthy at the expense of everyone else.

But he didn't stop there. He praised Obama's 2009 financial stimulus, his expansion of college aid, and his efforts to increase the use of renewable energy.

He unravelled Republican attacks on Medicare spending and welfare, and warned that proposed Republican cuts to healthcare would hurt the poor and disabled, not just the middle class.

And he tackled a question that initially flummoxed Obama aides when Republicans pressed it last weekend: Are Americans better off than they were four years ago?

"The answer is 'yes'," Clinton said. "But too many people are not feeling it yet."

Clinton's arguments are sure to be a template for other Democrats during the two months leading to the November 6 election, said Samuel Popkin, author of The Candidate: What it Takes to Win and Hold the White House.

"He did a better job of messaging Obama than the Obama people have done," said Popkin, a professor at the University of California in San Diego.

Democrats and Republicans alike say Clinton has few equals when it comes to explaining complicated subjects. Obama has had less success on that front.

Economists say his 2009 stimulus created millions of jobs but most voters view it as a failure.

Obama's Republican rival for the presidency, Mitt Romney, has explained his proposals to cut spending and simplify the tax code only in broad strokes.

Clinton argued that Romney's policies would either increase the national debt, shift more of the tax burden to the middle class or hollow out programmes such as those for child nutrition and school transport.

''Otherwise, the math doesn't add up," he said.

"I'm just a country boy from Arkansas and I came from a place where people thought two and two equals four," Clinton told the delegates.