Washington (dpa) - The presidential battle between Democrat Barack Obama and Republican John McCain has turned into a contest over who can lead the country out of a deluge of domestic and global crises.

A looming recession has become the dominant force in the race, overshadowing major foreign policy issues like the wars in Iraq and Afghanistan, Iran's and North Korea's nuclear activities, and a resurgent Russia.

Abroad, it is hoped a new president can rebuild America's image, tarnished by eight years of President George W Bush and the worst financial crisis since the Great Depression.

The election stands to make history on its own. McCain, at 72, would be the oldest president to begin a first term, while Obama, 47, would become the first African American in the White House.

Obama and McCain have duelled for the public's trust on who is best suited to stabilize the world's largest economy as voters, worried about dwindling retirement investments, high fuel costs and rising unemployment, head to the polls November 4.

In three debates and on the campaign trail, the candidates sparred over economic policies, but their arguments seemed based more on style and past records rather than significant differences over how to handle the financial slide.

Both senators backed the Bush administration's 700-billion-dollar rescue package for the finance industry aimed at thawing frozen credit lines and restoring confidence in the markets.

The sharpest differences came on taxes. McCain and Obama want more tax relief for the middle and working classes, but McCain accuses Obama of "socialist" tendencies for his plan to revoke the Bush administration's tax cuts on high income earners.

The economic collapse has further eroded Bush's public standing. His approval ratings have fallen below 30 per cent - the worst since Richard Nixon.

Obama has said a vote for McCain is a vote for more of Bush's policies, pointing to the Arizona senator's economic ideals of deregulation, plans and stance on Iraq, and charging that the country can't afford "four more years like the last eight."

McCain emphasized his long reputation as a "maverick," reformist- minded senator, touting his 20-plus years in Congress and expertise in foreign policy as making him the safer choice.

McCain has also not been shy about criticizing the current president and challenged Obama to stop comparing him to Bush.

"Senator Obama, I'm not President Bush," McCain said in a memorable debate moment. "If you wanted to run against President Bush, you should have run four years ago."

The economic crisis, coupled with Bush's unpopularity, has lifted Obama and his message of "change" from a razor-thin edge in August opinion polls to a significant lead during the campaign's closing weeks.

"There's usually more of a desire for change when people perceive things going badly," Frank Newport of Gallup Polling said in a telephone interview. "Obama has a lot going for him this year."

An average of polls compiled by realclearpolitics.com gave Obama 50.4 per cent to McCain's 43.1 per cent heading into the final week.

The mood might have swung to McCain's advantage had it not been for the economic crisis. He could have focused more on foreign policy and his strong support for Bush's troop buildup in Iraq, which has greatly reduced the violence from its highest level just two years ago.

Polls show voters have more confidence in McCain on foreign and defence policy. Obama's limited time in the Senate - under four years - and lack of experience are considered his biggest weakness. Obama selected veteran Senator Joe Biden, a foreign policy expert, as his running mate in an effort to counter that perception.

While McCain and Obama have pledged to take a more multilateral approach to foreign policy than Bush, they differ over key aspects of the war in Iraq.

McCain supported the 2003 invasion, while Obama, still not in the Senate, spoke out against it. Most Americans now believe it was a mistake despite the remarkable progress in the last year.

Obama plans to withdraw most US combat forces from Iraq within 16 months of taking office - a plan McCain calls "irresponsible." McCain, aligned with Bush, says the troops should stay in Iraq until the enemy is defeated or until Iraqi security forces are fully capable.

With this election's environment stacked against Republicans, McCain's lagging poll numbers prompted his campaign to take on a more negative tone, launching a series of attack ads. On the campaign trail, his vice presidential running mate, Sarah Palin, has led the assault on Obama.

For all the worldwide attention to the election, the outcome is likely to come down to a handful of "swing states" under the winner- take-all Electoral College. Even a narrow victory in a state ensures the winner gets all of the electoral votes - the number depending on the state's population.

Polls put Obama ahead in most of the usual major battleground states, including Florida, Ohio, Pennsylvania and Michigan. Demographic shifts have also helped Obama compete in some traditionally Republican regions, including southern states like Virginia and North Carolina and western strongholds like Colorado and Nevada.

Obama's edge has been aided by an unprecedented fundraising prowess. Over the course of the 20-month campaign, Obama raised more than 600 million dollars, nearly twice as much as McCain, according to the Federal Election Commission.

Obama's treasure chest comes mostly through the internet, allowing him to tap more than 3 million donors at an average donation of less than 100 dollars.

Turnout may be the biggest unknown in this election, but is expected to be higher, partly because of Obama's appeal to young voters, who typically have the lowest participation rate.

Obama's rallies at times attracted crowds reaching 100,000, while McCain's largest included about 25,000. But McCain's base lies in older voters who are more likely to head to the polls.