McCain in emergency donation plea
Mr McCain called for financial help to counter anti-Republican attacks
Republican US presidential hopeful John McCain has made a plea for emergency donations in an effort to pull off a surprise victory in Tuesday's election.
Mr McCain has been campaigning in Ohio - seen as a must-win state if he is to have any chance of overall victory.
The latest national poll by CBS and the New York Times puts him 13% behind his Democratic rival, Barack Obama.
Mr Obama held rallies in Virginia, Florida and Missouri on Thursday on a final dash around swing states.
Polls in battleground states suggest the differences between the two candidates are much slimmer.
Mr McCain said he would make a comeback in the final four days of campaigning.
"The pundits have written us off, just as they've done several times before," he said. "We're a few points down, but we're coming back."
But the BBC's North America editor Justin Webb says the latest surveys also suggest Mr McCain's running mate, Sarah Palin, is sinking in the public's estimation: Some 59% of respondents said she was not ready to be vice-president.
Meanwhile there were reports that Mr Obama had approached a controversial congressman to be his chief of staff if he becomes president.
The Illinois Congressman, Rahm Emanuel, is regarded as a highly partisan politician, and supporters of John McCain say he is an example of the real face of an Obama administration - governed from the left.
Mr McCain made his emergency donations plea less than a day after Mr Obama spent an estimated $5m (£3.1m) on a 30-minute primetime "infomercial" aired on US TV networks.
McCain has no wiggle room - one failure and he's done for
The BBC's Matthew Price, in Mentor, Ohio
In an email to supporters the Arizona senator wrote: "I'm asking for your financial support today to help us respond to attacks against our entire ticket.
"This election is not over and we need everyone's hard work in the coming days to be victorious."
On the campaign trail on Thursday, the candidates accused each other of favouring oil companies over middle class Americans, as oil company Exxon Mobil reported record profits.
At one campaign event in the town of Defiance, Mr McCain criticised his rival 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."
John McCain tries to introduce "Joe the plumber"
Mr McCain was joined at one rally by Joe Wurzelbacher - perhaps better known as "Joe the Plumber" - who has been adopted by the Republicans as a mascot after questioning Mr Obama's tax plan.
Mr Wurzelbacher had failed to appear at a previous campaign stop, despite being called on stage.
Mr McCain also restated his policy in favour of reducing US reliance on foreign oil imports.
"If I'm elected president, we're going to stop sending $700bn 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 has said that he backed the 2005 energy bill because it also contained tax breaks for renewable energy producers.
And the Illinois senator has accused Mr McCain of supporting more tax breaks for oil firms, because the Republican candidate favours cutting corporate tax rates.
Speaking in Florida, and later repeating the message in Virginia and Missouri, Mr Obama 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."
Former Democratic US Vice President Al Gore is scheduled to appear in Fort Lauderdale, Florida, with his wife, Tipper, later on Friday.
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.
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.