By Steve Schifferes
BBC News
The Obama campaign is set to broadcast a half-hour commercial across three major US networks on Wednesday.
The ad, which will air on CBS, NBC and Fox at 2000 (midnight GMT) on the East Coast, could cost up to $6m, it's been reported, which would make it the most costly single piece of political advertising in US history.
It is the first time since 1992 - when independent Ross Perot bought a series of 30-minute slots - that a candidate has chosen to use this format.
It has not been in common use for decades, because of the immense cost involved, and from 1992 until now no contender for the White House has been able to match Perot's extravagance.
"This is, in media terms, a rout. John McCain is in a shouting match against a guy with a megaphone"
"This is more than message imbalance," said Evan Tracey of the Campaign Media Analysis Group, which tracks political advertising
"This is, in media terms, a rout. John McCain is in a shouting match against a guy with a megaphone."
Fundraising edge
Mr Obama's extraordinary advantage in fundraising - he has raised nearly $660m, twice as much as Senator McCain - is being used to flood key states with TV advertising.
He is likely to spend more than $100m on TV advertising in October alone, more than Senator McCain's entire campaign budget, which is limited to $84m because he opted to accept federal funds.
Mr Obama is running 7,700 commercials per day, twice the number of the McCain campaign, and he can afford to run some spots of a minute or more, while all of McCain's commercials are 30 seconds long.
In recent weeks, the Obama campaign has also been able to afford costly national ads during NFL football matches, soap operas, and prime time shows.
Most of the political advertising in concentrated in a relatively small number of swing states.
Of the total spent by the Obama campaign so far, $150m has been spent in local TV markets. The $30m spent on national ads on network and cable TV dwarfs the $13m spent by the McCain campaign.
AD SPENDING IN KEY MARKETS
The top media markets for Obama campaign advertising were Philadelphia (Pennsylvania), Washington, DC (reaching Northern Virginia), Miami, Tampa and Orlando (Florida), Denver (Colorado), and Las Vegas (Nevada). There was also significant spending in the Mid West, including Detroit, Cleveland and Indianapolis.
The McCain campaign has nearly matched the Democratic spending in Denver and Las Vegas, and has also spent heavily in Pennsylvania, Ohio, Michigan and Minnesota.
Issues battles
The campaign TV ads are focusing on two key issues: taxes and jobs, and energy and the environment.
On taxes, the Obama campaign has given its 35 ads 205,000 showings, while 28 McCain ads have run 115,000 times.
But Mr Obama has also been able to give his 31 ads on job creation 126,000 run-outs, against 13 McCain ads which have run just 15,000 times.
He has also out-spent Mr McCain heavily on ads about healthcare, running over 100,000 spots on that issue, more than 10 times as many as his rival.
Experts say they have never seen such an imbalance in a presidential race before.
Negative advertising
Analysis by the University of Wisconsin Advertising Project shows that both candidates have used negative ads, despite Mr Obama's accusation, in the final presidential debate, that Mr McCain had run a wholly negative campaign.
The McCain camp did, it's true, use a higher proportion of attack ads.
In the four months to 4 October, 47% of Republican ads were negative (focusing on Obama), 26% were positive, and 27% were mixed, the Wisconsin study says. (For comparison, 70% of the ads used by George W Bush in 2004 were negative.)
On the Democratic ads, 35% were negative, 39% were positive, and 25% were mixed.
But the study points out that since the Democrats have aired twice as many ads, in total the amount of negative advertising is pretty even.
If American voters are feeling bombarded, it's not surprising. The presidential candidates are only paying for about half the advertisements screened in this election.
Overall, including ads focusing on the Congressional races, and ads paid for by special interests groups, the total bill for advertising is likely to exceed $1bn.
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