Former Vice-President Al Gore is preparing to campaign for Barack Obama in Florida, a state Mr Gore lost very narrowly in his 2000 presidential bid.
Republican US presidential hopeful John McCain has accused rival Barack Obama of offering oil companies tax breaks, at a rally in Defiance, Ohio.
Mr Obama's Republican rival John McCain is heading to the key state of Ohio.
The attack came as oil company Exxon Mobil reported record profits, and new figures showed US GDP shrinking.
On Wednesday evening, Mr Obama aired a half-hour, prime-time advert on seven US TV channels, before holding a rally with former president Bill Clinton.
Campaigning in Florida, Mr Obama said John McCain would continue the economic policies of President Bush.
Polls suggest Mr Obama is still ahead of Mr McCain in most swing states, and holds a solid lead in national polls.
Former Vice-President Al Gore will hold rallies for him on Friday in Florida, the state he narrowly lost in 2000.
Mr Gore is scheduled to appear in Fort Lauderdale, Florida, with his wife, Tipper.
'Corporate giveaways'
He lost the state of Florida in 2000 by just 537 votes, triggering a recount, which was controversially halted by the US Supreme Court.
At his Ohio campaign event, Mr McCain criticised Mr Obama for voting in favour of a 2005 energy bill, which contained tax breaks for oil companies.
"Senator Obama voted for billions in corporate giveaways to the oil companies," he said.
"I voted against it."
If you want to know where Senator McCain will drive this economy, just look in the rearview mirror Barack Obama
And he restated his policy in favour of reducing America's reliance on foreign oil imports.
"If I'm elected president, we're going to stop sending $700 billion a year to pay for oil from countries that don't like us very much. We're going to drill off shore and we're going to drill now."
Mr Obama, speaking at a rally in Florida, drew his supporters' attention to the news that US GDP had dropped by 0.3% in the third quarter of 2008, and blamed President Bush for the decrease.
"Our falling GDP is a direct result of eight years of the trickle-down, Wall Street first/Main Street last policies that have driven our economy into a ditch," he said.
"If you want to know where Senator McCain will drive this economy, just look in the rear-view mirror. Because when it comes to our economic policies, John McCain has stood with President Bush every step of the way."
Mr Gore is scheduled on Friday to appear in Fort Lauderdale, Florida, with his wife, Tipper.
Latest polls
The state holds significance for Mr Gore because - during his 2000 presidential run - he lost there by just 537 votes, triggering a recount, which was controversially halted by the US Supreme Court.
Had he won the state, he would have won the presidential election.
Had he won the state, he would have won the presidential election.
'Not perfect'
Mr Gore's visit follows a joint rally by Barack Obama and former president Bill Clinton in the state on Wednesday night.
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.
President Clinton told supporters they should not be complacent but try to persuade their friends and neighbours to vote for Mr Obama.
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.
"We have so much promise and so much peril," he said. "This man should be our president."
Bill Clinton joined Barack Obama on stage at a rally in FloridaIt 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.
'Great threats'
Later on Wednesday, Mr Obama appeared at his first joint rally with former US President Bill Clinton.
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.
To view this content you must have Javascript enabled and Flash player version 9 or higher installed.download Flash player
Mr McCain had also criticised 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.
Recent polls from CNN/Opinion Research suggest that Mr Obama leads Mr McCain by four points in Ohio, by six points in North Carolina and by seven points in Nevada, all states won by George W Bush in 2004.
Recent polls from CNN/Opinion Research suggest that Mr Obama leads Mr McCain by four points in Ohio, by six points in North Carolina and by seven points in Nevada, all states won by George W Bush in 2004.
The latest Rasmussen poll has Mr Obama leading Mr McCain nationally by 51% to 46%.
The latest Rasmussen poll has Mr Obama leading Mr McCain nationally by 51% to 46%.
Are you an American voter? Did you watch Mr Obama's TV advert? Were you impressed by what he said? Send your comment using the form below:
Are you an American voter? Did you watch Mr Obama's TV advert? Were you impressed by what he said? Send your comment using the form below:
