HENDERSON, Nev. (AP) — President Barack Obama has one mission heading into his first debate with Republican Mitt Romney: Don't screw things up.

Less than five weeks from Election Day, Obama has political momentum and an edge in polls of the battleground states that will determine the election. But he's sure to face a blistering challenge from Romney, who needs to use Wednesday's debate in Denver to change the trajectory of the race.

Both parties say the first debate traditionally helps the challenger, whose stature tends to rise in the eyes of many voters by simply appearing on stage as the alternative. Seeking to mitigate that effect, Obama aides are working with the president on keeping command of the debate while not being overly aggressive.