Barack Obama has promised to "change the world" if he is elected US President.

With just three days to go he and his opponent John McCain are touring key states in an effort to woo undecided voters.

Senator Obama is still almost seven points ahead in the Real Clear Politics poll of polls, but the gap has narrowed slightly.

At a rally in Henderson, Nevada, he warned his supporters against complacency.

"At this defining moment in history, you can give the country the change we need," he said.

Sky News' Michelle Clifford, who was at the rally, said Mr Obama was trying to leave nothing to chance.

"He'll be using every ounce of his resources to get the vote out," she said.

At the same time Senator McCain was rallying his followers in Newport Beach, Virginia.

In a usually safe Republican state, which is threatening to go to the Democrats, he asked for help on the home stretch.

He said: "Let me state the obvious again, we need to win Virginia on the 4th of November and with your help we're going to win and bring real change to Washington.

The campaigning has been tough for both men, but Sky News' Robert Nisbet, who has been following the McCain bandwagon, says the toll is beginning to show on the older man.

"Being on the road at rally after rally is exhausting and Mr McCain appears to be tired," he said.