By Andy Sullivan

WASHINGTON, Sept 30 (Reuters) - The debate between President Barack Obama and Republican challenger Mitt Romney on Wednesday marks the first time the two candidates will be able to challenge each other directly on the economic issues that have been the focus of the presidential campaign.

Viewers should be able to determine how each candidate fares by keeping an eye on the following five factors:


* ROMNEY ON OFFENSE, OBAMA ON DEFENSE

With less than six weeks to go until the election, Romney is under pressure to deliver a performance that shifts the momentum in his direction.

Obama, on the other hand, merely needs to avoid a catastrophic performance that could cause independent voters to reassess their support.

Both are experienced and competent debaters, but neither appears to enjoy the give and take that occurs at these events.

For each candidate, the challenge will be to rattle their opponent enough to prompt an off-script outburst.

"Obama just wants to avoid any big mistakes. Typically candidates are undone more by their own mistakes than by the successes of their opponents, the witty ripostes or devastating one liners of their opponents," said George Washington University political science professor John Sides.

"For Romney, there's more pressure and he really needs the debate to change the dynamic of the race."


* EYES NEVER LIE

Television is a visual medium, and the body language of the candidates can have a bigger impact than their words.

Democratic Vice President Al Gore's repeated sighs in a 2000 debate with George W. Bush turned voters off, while Bush drew negative attention in 2004 when he scowled while his Democratic opponent John Kerry spoke. Bush's father, President George H.W. Bush, looked at his watch in a 1992 debate, a move that many interpreted as impatient and aloof.

Obama and Romney want to avoid obvious missteps like these, but more subtle signals can also signal to viewers that candidates aren't on the level.

Shoulder shrugs indicate uncertainty, a wrinkled upper lip signals disgust, and eye blinking, either too much or too little, can convey stress, said body-language expert Janine Driver, author of "You Can't Lie to Me."

On the other hand, a candidate conveys confidence when he turns his body to face his opponent.

"We'll see them face each other when they think they're going to knock it out of the park," Driver said. "I call it 'navel intelligence.'"


* WHO WINS THE FIRST ROUND?

Alert viewers will be able to get a sense of how the debate will play in the news media by watching the first 30 minutes closely, although the impact of the debate probably won't register in opinion polls until several days after the event.

Candidates need to establish their themes and launch their most important attacks early in the debate, while reporters and analysts are still forming their impressions about how the debate is going, according to former Gore adviser Ron Klain.

"While you can lose a debate at any time, you can only win it in the first 30 minutes," Klain wrote in a memo for the centrist Democratic think tank Third Way.