Republican John McCain headed for must-win Ohio with just five days to overcome Barack Obama.
Mr McCain said the race grows tighter each day, but he faces long odds against Mr Obama's well-oiled campaign machine. According to most state polls, Mr Obama has surged past the 270 electoral votes needed to capture the White House.
And not since Ross Perot's third-party candidacy 16 years ago has a candidate had the money to finance the kind of nationally broadcast and lengthy message Mr Obama put on American airwaves in prime television time.
Mr McCain, a veteran senator who stumbled badly in the early days of America's historic economic unravelling and has been unable to shake perceived ties to unpopular President George Bush, derided the event as a "gauzy, feel-good commercial," paid for with broken promises.
He appeared as a guest on CNN's Larry King Live after claiming earlier that Mr Obama lacked "what it takes to protect America from terrorists."
In Mr Obama's broadcast, the candidate was shown addressing large crowds on issues including health care, education and jobs, interspersed with the story of struggling American families, and interviews with Mr Obama's family and colleagues.
Mr Obama talked about how his mother died young of cancer. "I know what it's like to see a loved one suffer, not just because they are sick, but because of a broken health care system."
He also pledged to protect the US while seeking to wind down the war in Iraq.
"I will not be a perfect president," Mr Obama said. "But I can promise you this - I will always tell you what I think and where I stand."
As the commercial ended, it cut to live shots of an Mr Obama rally in Florida, where the candidate was shown with his running mate, vice presidential candidate Joe Biden.
Mr McCain dismissed the Obama broadcast as a "TV special".
"As with other infomercials, he's got something to sell you," Mr McCain said. "He's offering you government-run health care."
