Mr Obama's ad went out on all but one of the major US TV networks
Presidential candidate Barack Obama has reached out to US voters in a half-hour, prime-time advertisement on seven US TV channels six days before polling.
The broadcast came as the Democratic senator appeared with former president Bill Clinton at a rally in Florida.
Meanwhile Republican John McCain has launched an attack on Mr Obama's economic and security abilities.
Mr McCain said he is not qualified to act as commander-in-chief and would raise taxes.
Senator Obama leads in national and most swing-state opinion polls.
Mr Obama's half-hour TV spot was shown on CBS, Fox and NBC, at a cost of about $1m (£630,000) per network, as well as on Spanish language channel Univision, BET, MSNBC and TV One.
The advertisement featured interviews with Americans talking about difficulties in their lives, as well as clips of various political and business figures saying why they supported Mr Obama.
Bill Clinton joined Barack Obama on stage at a rally in Florida
It showed footage of Mr Obama on the campaign trail, outlining his policies and talking about his background.
There was no mention of Mr McCain or the Republican Party.
"I will not be a perfect president," said Mr Obama. "But I can promise you this - I will always tell you what I think and where I stand."
The broadcast ended with Mr Obama speaking live from a rally in Florida, where he urged his supporters to continue to campaign on his behalf in the final days before the election.
The BBC's Justin Webb in Washington says that while it is not unheard of for American politicians to buy large chunks of television time - millionaire Ross Perot did it in 1992 - Mr Obama's move is unprecedented in its scope.
Only one of the major TV networks, ABC, did not run the film - which has been weeks in the making - and Fox News decided not to broadcast its pre-game show ahead of the fifth game of the baseball World Series in order to accommodate it.
Later on Wednesday, Mr Obama appeared at his first joint rally with former US President Bill Clinton.
The question is whether this is a man who has what it takes to protect America
Mr Clinton, whose wife Hillary was defeated in the race for the Democratic candidacy, praised Mr Obama, saying he had the ability to make decisions and bring about change.
He told supporters at the Florida rally they should not be complacent but try to persuade their friends and neighbours to vote for Mr Obama.
"We have so much promise and so much peril. This man should be our president," said Mr Clinton.
In a speech to supporters in Tampa, Mr McCain had questioned his rival's security credentials, saying: "The question is whether this is a man who has what it takes to protect America from Osama bin Laden, al-Qaeda and the other great threats in the world."
"He has given no reason to answer in the affirmative".
Mr McCain's assault on his opponent's defence and security policies came after a meeting with a number of senior former military officials.
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Mr McCain had also launched a series of attacks on his rival's tax policies, while at a campaign event in Miami.
"This is the fundamental difference between Senator Obama and me," he said.
"He thinks taxes are too low, and I think that spending is too high."
Mr Obama, at an event in North Carolina, poured scorn on Mr McCain's recent accusations that he is a "socialist".
"By the end of the week, he'll be accusing me of being a secret communist because I shared my toys in Kindergarten," he told supporters.
The latest Associated Press-GfK poll has Mr Obama and Mr McCain virtually tied in Florida and North Carolina. It has Mr Obama leading Mr McCain in Colorado, Nevada, New Hampshire, Ohio, Pennsylvania and Virginia.
The latest Gallup poll has Mr Obama leading Mr McCain nationally by 50% to 43%.
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