On Thursday McCain's camp in turn accused Obama of playing "the race card" after the Illinois senator said Republican attacks were "trying to make you [the US voter] scared of me".

"Barack Obama has played the race card, and he played it from the bottom of the deck. It's divisive, negative, shameful and wrong," said McCain's campaign manager Rick Davis in a statement.

Obama had remarked that the attacks say "he's not patriotic enough, he's got a funny name ... he doesn't look like all those other presidents on those dollar bills, you know, he's risky".

'Dishonourable campaign'

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McCain's camp defended the "celebrity" television advertisement and said that Obama's associates were overreacting.

"It celebrates the excitement that he has generated, that is certainly more akin to the excitement that a celebrity generates than a normal politician," Nicolle Wallace, a senior adviser to McCain, told MSNBC on Thursday.

However, Robert Gibbs, an adviser to Obama, said McCain was running "an increasingly dishonourable campaign".

"The McCain campaign has very clearly decided that the only way to win this election is to become very personal and very negative," he said.

"We believe that people will see that as nothing more than the same old politics and the same old policies of the last eight years."

Pop star

John Nichols, a political writer at The Nation magazine, told Al Jazeera McCain's advert had tapped into concerns in the US that Obama was an unknown quantity.

"He [Obama] seems to have exploded across the scene like a new pop star or a new movie star," Nichols said.

"It's been good for McCain in that he has captured part of the public discourse [about Obama] and at this point thats what he needs," Nichols said.

If elected to the White House Obama would be the first African-American US president.

The contest between the two men ahead of presidential elections on November 4 has so far not dwelt on the often explosive issue of race in the US, although some accused associates of Obama's former Democratic presidential rival, Hillary Clinton, and her camp of exploiting race for votes.

Both candidates have previously said they planned to run campaigns that would avoid the negative attacks that characterised some presidential elections in the past, notably between George Bush and John Kerry in 2004.