John McCain, the US Republican presidential candidate, and his Democratic rival, Barack Obama, have continued their battle for votes with only four days left before polls open.

Arnold Schwarzenegger, the governor of California, put his star power behind McCain as the Republican campaigned for a second day in the battleground state of Ohio.

Schwarzenegger told crowds in Columbus that Obama's four years as a senator from Illinois was nothing compared to the five-and-a-half years McCain spent in a Vietnam prisoner of war camp.

"John McCain has served his country longer in a PoW camp than his opponent has served in the United States Senate," he said.

"Ladies and gentlemen, I only play an action hero in my movies - but John McCain is a real action hero," he said.

Voters' fears

Al Jazeera's Teresa Bo, reporting from Columbus, said: "Schwarzenegger came out and showed his support for McCain, as he did for George Bush back in 2004."

She said the former movie star continued the McCain campaign's strategy of playing on voters' fears over the beleaguered economy and taxation.

"He told people that Obama was a socialist who was going to raise taxes," she said.

Meanwhile Obama held a rally in Des Moines in the state of Iowa before visiting Indiana, a state where the candidates are currently tied.

In Des Moines, he again attacked the Republican party's record on the economy.

"The last thing we can afford is four more years where no one in Washington is watching anyone on Wall Street because politicians and lobbyists killed common-sense regulations," he said.

"Those are the theories that got us into this mess. They haven't worked, and it's time for change."

'Slash and burn'

He warned his supporters to expect to see from the McCain camp: "... more of the slash and burn, say-anything, do-anything politics, throw everything ... and see if anything sticks, a message that's designed to divide and distract; to tear us apart instead of bringing us together."

Al Gore, the former Democratic vice-president and presidential candidate, campaigned on Obama's behalf in Florida, the state where his defeat against George Bush, the current president, cost him the 2000 election.

He told Obama supporters: "We've been borrowing money from China to buy oil from Saudi Arabia and burn it in ways that destroy the future of human civilisation.

"We've got to change every single bit of that - and we have a chance in this election to strike out on a new course."

The latest campaign events come as a poll released on Friday showed Obama leading McCain nationally by 51 points to 43 per cent.

However, the survey, by Associated Press-Yahoo News, also found that 14 per cent of voters still cannot decide who they want to vote for or may switch their vote on November 4.

Record advertising

Analysts say Obama has widened his lead in recent weeks by his perceived better handling of the current financial crisis, and the two candidates have clashed repeatedly in recent weeks over the issue.

Meanwhile, a study released on Thursday found that Obama has spent three times more on television advertising than his Republican rival in the final days of the campaign.

Obama spent nearly $21.5 million on TV ads between October 21 and 28, while John McCain's campaign spent close to $7.5 million, the University of Wisconsin Advertising Project said.

Obama's figure does not include the approximate $3 million that Obama's campaign spend on 30-minute advertisements on national television.