Both men have raised the temperature of the campaign
They promised positive, high-minded campaigns which focused on issues rather than personalities.
But as the race for the White House intensifies, the two contenders have increasingly gone on the attack.
Republican John McCain has earned huge publicity for an advert mocking rival Barack Obama as "the biggest celebrity in the world", juxtaposing the Democrat with gossip magazine regulars Britney Spears and Paris Hilton.
But Senator Obama has not hesitated to lambast his opponent and his allies as "cynical", "desperate" and "in the pocket of Big Oil".
Negative campaigning is, of course, hardly a new phenomenon in US politics and, as Professor Shanto Iyengar of Stanford University attests, it is often the quickest way for politicians to grab attention.
"That is a stock approach here - candidates use controversial personal attacks as a means of attracting free news coverage," he says.
Senator McCain has so far been the more forthright of the two contenders when it comes to going on the offensive.
The Republican has accused his opponent, who promises to withdraw troops from Iraq, of being prepared to "lose a war in order to win a political campaign".
A McCain campaign advert also charged Mr Obama with cancelling a visit to injured US soldiers.
But in the blue corner, Mr Obama has aimed regular blows at his rival.
John McCain's latest campaign ad and Barack Obama's response
He has accused Mr McCain of taking the "low road" in the election and dismissed his time in the US Senate as "years of inaction".
An Obama advert accused Mr McCain of practising the "policies of the past", using images of him with President George W Bush.
The McCain camp also protested when John Kerry, the Democratic candidate in 2004, said the 71-year-old was "confused" over Iraq.
The tone of the contest took an angrier turn after Mr Obama warned an audience that the Republican would try to scare voters about how he looked unlike "all those other presidents on the dollar bills".
A McCain aide responded furiously, attacking Mr Obama for having "played the race card" - all previous US presidents having been white men.
Mr Obama said no-one had thought his comments were accusations of racism until Mr McCain's team "started pushing it".
If Mr Obama has so far avoided the jugular, it is only because his campaign team do not want to tarnish his brand of idealism and hope, Professor Larry Sabato of the University of Virginia believes.
"The candidate of fresh, optimistic change cannot afford to go too negative," Mr Sabato says.
"McCain is also now given more leeway to go negative since he is the underdog. It isn't fair, but nothing about politics is fair."
Few American voters are likely to be shocked by such tactics. Negative campaigning has, after all, played a key role in modern elections.
The Swift Boat Veterans for Truth - who questioned Mr Kerry's Vietnam war record in the 2004 presidential election - were widely agreed to have contributed to George W Bush's victory.
In 1988, the "Willie Horton" campaign run by George Bush against Michael Dukakis accused the Democrat of supporting weekend leave for prisoners who went on to reoffend.
The famous Daisy ad played on Cold War fears of a nuclear exchange
And perhaps the most famous negative advert of all time was shown just once in 1964.
Lyndon Johnson's "Daisy" showed a small girl counting as she picked the petals off a flower. When she reached nine, an ominous adult voice counted down to 10 and the scene dissolved into a nuclear explosion - playing on public fears that Mr Goldwater would start a war if elected.
Not everyone believes that negative campaigning is intrinsically unhealthy.
Brooks Jackson, director of the non-partisan campaign monitoring group FactCheck.org, says he is less concerned by whether an advert is positive or negative than by whether it is accurate.
"Political scientists and communications scholars will tell you that negative ads tend to have more information in them than positive ads," he says.
"We see positive ads that are false or misleading, too."
Professor John Geer of Vanderbilt University, Tennessee, is the author of In Defense of Negativity - a spirited defence of the role of attack adverts in the political process.
He says it is vital that candidates are held up to scrutiny and examined under fire.
"Being president is a tough job - ask Bush or Clinton," he adds.
"Having a negative campaign run against you is a test of your toughness. You come out of the election battle-tested."
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