Los Angeles - From health care to taxes, Iraq to Iran, Joe Biden to Joe the Plumber - the candidates in Tuesday's presidential election are blitzing the US airwaves with an avalanche of ads.

It's their final chance to persuade voters at the climax of the most expensive campaign in electoral history, and Democratic nominee Barack Obama holds the advantage.

Historians will no doubt note that the candidates and their parties raised and spent close to 1 billion dollars on their campaigns just as country and the global economy are mired in the worst financial crisis in memory.

Obama enjoys a potentially decisive edge, thanks largely to an incredible grassroots fundraising campaign has out raised and outspent Republican candidate John McCain by a factor of as much as two to one.

This advantage has been especially pronounced in what many regard as the three most important battleground states of the election. According to the media tracking firm Nielsen, Obama has outspent McCain in Florida by 232 per cent. In Ohio the difference is 162 per cent, and in Pennsylvania Obama's advantage is merely double.

Obama is running 7,700 commercials per day, according to the Campaign Media Analysis Group, twice the number of the McCain campaign, and some of those spots are a minute or more, while all of McCain's commercials are just 30 seconds long.

The culmination was the 30-minute appeal his campaign ran on all but one of the major broadcast TV networks Wednesday night, at an estimated cost of 6 million dollars. It was the first such ad buy since billionaire independent candidate Ross Perot undertook a similar gambit in 1992.

The long ad featured campaign clips of Obama speaking to huge crowds, interspersed with footage of him speaking directly to viewers, in between vignettes about working-class supporters struggling to make ends meet. He asked voters to "choose hope over fear and unity over division"

Obama's campaign said that the spot gave their man the opportunity to address wavering voters directly and clinch the election. The McCain campaign, which is financially unable to match Obama's ad spending, dismissed the move as another example of Obama's celebrity mindset.

McCain has unleashed a barrage of traditional 30-second spots designed to portray Obama as an inexperienced leader whose international policies would embolden US enemies abroad. At the same time, the ads have tried to link Obama with "socialist" ideals of "tax and spend," a line of attack adopted after the centre-left candidate suggested a need to "spread the wealth."

One ad pictures fearsome-looking members of the Iranian Revolutionary Guard and argues that Obama is "dangerously unprepared to be president." Others show disturbing, black-and-white pictures of rolling tanks and frightened children, and harp about Obama's lack of experience.

One ad shows Obama waving to adoring crowds.

"Behind the fancy speeches, grand promises and TV specials lies the truth - Barack obama lacks the experience America needs," intones a grave commentator. "The fact is that Barack Obama is not ready ... yet."

Obama is not above using the horror-movie music and black-and- white images to scare people away from his opponent as "out of touch, out of ideas and running out of time." Another quotes McCain as admitting that he doesn't know enough about the economy and might have to rely on his vice president in that respect, before cutting to a picture of a winking Sarah Palin.

Mostly, Obama's TV spots show the candidate himself looking straight into the camera and detailing policies that he says will make a difference in the everyday lives of Americans.

He has stuck to a tactic of quickly responding to McCain's attacks. For instance, McCain's "Joe the Plumber" ads alleging that Obama plans tax hikes are often sandwiched between ads from Obama touting tax breaks for ordinary Americans.

"Any way you look at it, Obama has a huge advantage," says advertising executive Clark Bigham. "His message is reaching the people where it matters and is making it harder than even for McCain to come back in the last, final days of the campaign." (dpa)