CHICAGO (AP) — Six weeks from Election Day, President Barack Obama's campaign has momentum — and a big case of nerves.

Top advisers are both relishing in Obama's edge in key battleground state polling and warning it can change in an instant.

They're wary of the many factors that still could derail the Democrat's campaign, from simmering tensions in the Middle East to three high-stakes presidential debates. They're worried, too, about a flood of negative advertisements from Republican-leaning outside groups and potential complacency among Democratic voters and volunteers who think the race is a lock for Obama.

Also weighing on them: unforeseen domestic or international events that can shake up a close race in the homestretch.

Public and internal polls show Obama leading Republican Mitt Romney in many key battleground states.