Barack Obama and John McCain clash on Friday in the first of three crucial one-on-one debates, which have the potential to make or wreck their rival challenges for the White House.

The foes will clash at a rare moment of national peril, with the staggering US financial system spawning a global crisis, the stock market reeling and the life savings of millions of Americans in the balance.

Tens of millions of television viewers are expected to tune into the contest in Oxford, Mississippi at 9:00 pm (0100 GMT) on Friday, five weeks before election day.

In 2004, more than 62 million people watched the first debate between President George W. Bush and John Kerry -- given the historic potential of this year's gripping election race, analysts expect an even bigger audience.

Following an agreement between the campaigns, the first debate will focus on foreign policy and national security, an area where polls show Republican McCain holds an advantage over first term Democratic Senator Obama.

But both candidates are likely to brandish their economic credentials as the Bush administration readies the biggest economic rescue package in decades.

Conditions best for Obama

With the economy teetering, US troops mired abroad in two wars and a highly unpopular Republican president, all conditions should benefit Obama.

But the Democrat, 47, still has not closed the deal and so he faces intense pressure at the debates to convince voters he is ready to serve as president.

Democratic pollster Celinda Lake said a George Washington University poll last week reveals Obama's task.

"McCain has a 14 point advantage on being a strong leader, and among independent voters that jumps to a whopping 26 percent advantage," she said.

"I think the debates will be very, very important in this regard.

"They will be the first time that these two stand as co-equals and that is a real proving ground for Obama to show that he is a leader and a strong leader equal in stature to McCain."

Obama goes into the debates on a surge of momentum, after recapturing the lead in national polls: a Gallup daily tracking poll on Saturday gave him a 50 to 44 percent lead among registered voters.

McCain has endured a rough week, struggling to frame a consistent message on the crisis and trying to evade Obama's attempts to saddle him with the Bush administration's unpopular economic legacy.

Debates crucial

But analysts said a strong performance by either candidate in the debates, or a string of gaffes, still have the potential to remake the race in the crucial final stretch up to the November 4 election.

"There will be a significant number of people tuning in for the first time," said University of Arkansas political scientist Andrew Dowdle.

"The images of the candidates still seemed to be unformed, and elastic at this point."

In a presidential debate, the demeanor of the candidate is often as important as what they say.

McCain will hope to show up Obama as inexperienced, and not ready to assume the duties of commander in chief at a time of global peril.

But he is under intense pressure not to show any hesitation, tiredness or confusion that would remind voters that at 72, he would be the oldest president inaugurated for a first term.

Obama will try to rein his more professorial instincts, and aim for snappy, and precise answers.

History shows crucial role

Presidential debates have often played a crucial role in the outcome of elections.

Democrat John F. Kennedy appeared young and dynamic in 1960, outpointing his rival Richard Nixon and emerging victorious in November.

In 1980, Ronald Reagan used the debates to pass the test of presidential credibility and went on to beat president Jimmy Carter.

Four years later, he put to rest questions about his advanced age -- swatting challenger Walter Mondale with the immortal line: "I am not going to exploit for political purposes, my opponent's youth and inexperience."

In 1992, George Bush looked at his watch during a debate with Bill Clinton -- adding to the impression he was oblivious to the struggles of normal Americans.

And Democrat Al Gore's repeated sighs in a melodramatic performance in 2000, handed the initiative to then Texas governor George W. Bush.

After Friday, McCain and Obama will meet in two more debates, on October 7 and 15. Their vice presidential running mates Sarah Palin and Joseph Biden will go head-to-head in a single debate on October 2.