John McCain has made a plea for emergency donations in an effort to pull off a surprise victory.
Mr McCain has been campaigning in Ohio — a must-win state if he is to have any chance of overall victory.
The latest national poll by CBS puts him 13% behind his Democratic rival, Barack Obama who this week spent £2.4m on a 30-minute primetime TV ad.
McCain and his Straight Talk Express campaign bus will roll through the key state of Ohio today as the US presidential election enters its final four days.
Seen as a microcosm of modern America, the Buckeye State has voted with the winner in every presidential election for two decades.
But the latest polls suggest it will be an uphill battle for the Republican.
Just 127 electoral college votes, of the 270 needed to win the presidency, are considered to be in safe Republican hands, compared with the 238 considered safe for the Democrats. A further 15 are considered to be leaning towards Mr McCain, with 73 leaning towards Mr Obama.