The United States economy has been laid low by shrinking growth, bounding unemployment and staggering increases in energy costs.

While Mr Obama sets off with an offensive against Mr McCain on the economy, the latter hopes to counter-punch with attacks on his Democratic opponent's perceived lack of experience in the international arena.

However, Mr McCain is saddled with the Iraq war, which he supported at its inception and believes must continue to a still undefined US victory. But for now, the issue has fallen off front pages as voters turn inward, concerned about their faltering economic prospects.

The national average price of regular grade petrol crept up to US$4 (S$5.40) a gallon (3.8 litres) for the first time at the weekend, passing the once-unthinkable milestone. The country's unemployment rate also shot up to 5.5 per cent last month, the biggest one-month jump in decades.

Opinion polls indicate more than 80 per cent of Americans believe the country is going in the wrong direction - a signal of a problem for the party in power.

'The atmospherics for the Republicans have not been this bad since 1974,' said former McCain adviser John Weaver.

The focus on the economy signals that Mr Obama has identified voters' fears over job losses and rising prices as the key potential battleground in November.

And Mr McCain will need all the financial help he can muster as he moves towards the November vote against Mr Obama's well-oiled fund-raising machinery.

Mr Obama, 46, has raked in US$264 million in 16 months. Mr McCain has raised less than half that much - US$115 million, in 17 months.

Yesterday, Mr Obama was launching his two-week, nationwide economic tour from North Carolina - a state that has not voted for a Democratic presidential hopeful since 1976.

The Illinois senator plans to push into other Republican bastions with his economic message, hoping to reverse history in places such as Missouri, which has not voted for a Democrat since 1996, and having visited Virginia last week which last voted Democratic in 1964.

By moving into such historically Republican states, Mr Obama could force the McCain camp to spend precious resources defending what is traditionally seen as home turf.

Mr McCain has planned a series of fund-raising stops as the general election campaign begins in earnest now that Mrs Hillary Clinton has suspended her bid for the Democratic nomination and robustly thrown her support behind Mr Obama at the weekend.

Both candidates were also rolling out biographical television spots to re-introduce themselves to voters after the gruelling primary season.

Mr Obama's story is of a mixed-race trailblazer who, he says, personifies hope and the American dream.

Mr McCain, 71, is that of the grizzled veteran and war hero who survived five years of torture during captivity in Vietnam.

ASSOCIATED PRESS, AGENCE FRANCE-PRESSE