The Republican Party sent an automated telephone call on Sunday to millions of voters, which featured a recording of Hillary Clinton mocking Obama's lack of experience when she was running against him for the Democratic nomination.
Clinton said in the recording: "In the White House, there is no time for on-the-job training. Senator McCain will bring a lifetime of experience to the campaign and Senator Obama will bring a speech that he gave in 2002."
The Republicans are hoping the "robocalls" will sway undecided voters to vote for McCain, the 72-year-old war veteran who entered politics in 1981.
Clinton, who has been campaigning on behalf of Obama since her defeat, is reportedly furious at the use of her remarks.
Kathleen Strand, a Clinton spokeswoman, said: "Senator Hillary Clinton does not approve this message and as she criss-crosses the country, she has said time and time again that the choice in this election could not be more clear.
"The McCain/Palin ticket offers only more of the same failed policies, while the Obama/Biden ticket offers the vision, leadership and positive solutions we need. I wonder why the Republicans aren't using those words?"
Obama hit back at McCain by comparing his own backing from billionaire investor Warren Buffett and former US Secretary of State Colin Powell to McCain's last-minute endorsement by the unpopular vice-president Dick Cheney.
Separately, CNN International has launched a viewer comment initiative to encourage students worldwide to provide their opinion on the 2008 US presidential election.
The "World View" initiative invites users to upload self-authored video telling others why the election is important to them and their home country.
Mark Haviland, director of marketing at CNN International, said: "The story we have here, in addition to an amazing political race, is one of people's reaction to this election from around the world, and we are making sure as many as possible have a chance to get involved."
The initiative will be marketed on social networking site Bebo, as well as through outdoor, print, online and direct mail ads.
Meanwhile, outdoor ad firm Clear Channel Outdoor will be using its digital billboard networks to provide the US public with real-time tally updates and final results of the election through live RSS-feeds from msnbc.com.
250 Clear Channel digital billboards and 300 Digital Smart Tops on New York and Boston based taxis will show motorists, commuters and pedestrians the results of the electorate and popular vote.