(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." [Reuters]