By Philip Sherwell in Oxford, Mississippi Last Updated: 7:58AM BST 28 Sep 2008
Two instant television polls and a focus group conducted by top consultant Frank Luntz gave the Democratic senator a lead over his Republican rival among the all-important undecided voters.
His apparent victory was not clear cut, however, with some pundits declaring Sen McCain the winner on points just five weeks before Americans cast their ballots.
Although the party nomination battles began 20 months ago and the election has already cost more than $1 billion, this was the first time that many Americans will have focused closely on the performance of the two candidates to replace President George W Bush.
With two more debates due before the Nov 4 voting, neither candidate landed a knock-out punch or committed the sort of disastrous gaffe that can determine an election.
"There are some advantages to experience and knowledge and judgement," he said. He then taunted Mr Obama by quoting a remark used in the primaries by Joe Biden, who subsequently became the Democratic senator's running mater. "I don't need to do any on-the-job training," he said.
In turn, Mr Obama repeatedly linked the Arizona senator to the failed policies of the Bush administration, saying Mr McCain had agreed with the president "90 per cent of the time".
When pressed to answer the most important question in America today, however, neither candidate was willing to risk a clear response: they both dodged questions on the $700 billion plan to rescue Wall Street.
Mr McCain cited his battle against wasteful federal expenditure, the first of many references during the evening to his "record" - drawing an implicit contrast with his rival's inexperience on the national stage.
Mr Obama countered by seeking to tie Mr McCain to the economic policies of the Bush White House and its "orgy of spending" and argued that he was out of touch with the needs of American workers.
During the exchanges on economics, Mr McCain accused Mr Obama of having "the most liberal voting record in the Senate" and then added: "It's hard to reach across the aisle from that far to the left."
Mr Obama responded: "John mentioned me being wildly liberal. Mostly, that's just me opposing George Bush's wrongheaded policies since I've been in Congress."
In these televised debates, as much attention is paid to style as substance. Mr Obama still came across as cool and slightly detached at times - although not as aloof or professorial as during the primaries. And he came to life during the more lively clashes over foreign policy.
But Mr McCain also sometimes reinforced negative impressions of himself as a "cranky old man" as he repeatedly put his young foe down with the words "he doesn't understand" and refused to look him in the eye.
Just hours before the debate began it was unclear whether it would happen at all. Mr McCain had stunned Americans by announcing he would not take part in order to help push the financial bail-out through Congress.
But at the last minute he changed his mind and flew to the small college town of Oxford where the debate was being held on the campus of the University of Mississippi.
While the financial crisis dominated headlines, it was foreign affairs that provoked the sharpest exchanges in Friday night's showdown.
On Iraq, Mr McCain assailed Mr Obama for opposing the recent troop "surge", refusing to acknowledge its success, insisting on a timetable for withdrawal and not visiting the country for more than 900 days.
But Mr Obama took the fight to Mr McCain, reminding the audience that he had opposed the war from the start and then attacking his rival's judgment on a series of key issues.
"You said it was going to be quick and easy. You said we knew where the weapons of mass destruction were. You were wrong. You said we were going to be greeted as liberators. You were wrong. You said that there was no history of violence between the Shia and Sunni. You were wrong."
Mr McCain took the harshest digs at Mr Obama over his assertion during the primary battle with former First Lady Hillary Clinton that he would be willing to meet the Iranian president Mahmoud Ahmadinejad without setting conditions.
"Sen Obama doesn't seem to understand that if without precondition you sit down across the table from someone who has called Israel a 'stinking corpse', and wants to destroy that country and wipe it off the map, you legitimise those comments," he said witheringly. "This is dangerous. It isn't just naive, it's dangerous."
Mr McCain also slammed Mr Obama for allegedly saying he would attack Pakistan. That brought a stinging riposte. "Coming from you, who in the past has threatened extinction for North Korea and sung songs about bombing Iran, I don't know how credible that is," said Mr Obama.
The Democrat repeatedly said that Mr McCain had backed Mr Bush in making Iraq a priority when Osama bin Laden remained free somewhere on the Pakistan-Afghanistan border.
Although the debate was originally scheduled to cover foreign affairs and national security, the first 40 of the 90 minutes were eventually allotted to the economic crisis.
Using hand-held dials, they indicated their reactions throughout the debate. Thirteen had supported Democrat John Kerry four years ago and 12 were for Mr Bush, with two voting for neither. By the end of Friday's debate, 17 said they felt more favourable about Mr Obama and 10 about Mr McCain.
"They felt that McCain was too negative and they didn't see the validity of some of his attacks," Mr Luntz told The Sunday Telegraph. "They felt he had the experience ut they wanted to hear him talk about the future not the past. And they felt he had been playing politics when he threatened not to turn up for the debate.
"Obama came across as more passionate and more eager. He seemed to have more life to him.
"It was an ok night for John McCain and a good one for Barack Obama. The trouble for McCain is that he's the one behind in the polls. He now only has two debates left to score."
In a so-called "insta-poll" of 524 uncommitted voters for CNN, Mr Obama won the debate by 51 per cent to 38 per cent. CBS conducted a similar survey with a victory for Mr Obama by a 39 to 24 per cent margin, with 36 per cent declaring it a draw.
Mr McCain's campaign senior strategist Steve Schmidt stuck to similar talking points, arguing:
"McCain showed his mastery of the issues tonight and Obama was on the back foot. Sen Obama is a gifted speaker but he doesn't have a record of bringing about change."
"Only one candidate was presenting a vigorous case for change and standing up for real America. That was Barack Obama. McCain is mistaking his long resume for evidence of wisdom and judgement."