barack-obama-3_web.jpgmitt_romney_8.jpgCHICAGO — Six weeks from Election Day, President Barack Obama's campaign has momentum — and a big case of nerves.

Top advisers are both relishing Obama's edge in key battleground states 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 the 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.

“I can only worry about what I can control,” says Jim Messina, Obama's campaign manager.

For now, that means the overall campaign strategy.

Yet, even there, Obama's team is being careful: The Democrat is considering making a late play for traditionally Republican Arizona — either to win it or to force rival Mitt Romney to spend money to protect GOP turf — but advisers are weighing the potential that a move like that could backfire by leaving fewer resources for more competitive states.

With just over 40 days until the election and with many states already voting, public and internal polls show Obama leading Romney in many of the eight or so battleground states that will determine the election.

But both campaigns are mindful that much can happen in the home stretch and advisers for each candidate expect the numbers to tighten as more voters tune to the race in the final weeks.

By the day, both sides are adjusting their strategies in key states and monitoring how, in voters' eyes, signs of growth in the economy square with an unemployment rate that remains above 8 percent.

The race had been deadlocked until recently when Obama edged ahead in polling after his convention. Even so, neither candidate has been able to put the race out of reach of his opponent, despite the sluggish economy Obama has presided over and a series of missteps by Romney.

At Obama's Chicago campaign headquarters, the mood underscores the balance the campaign is trying to strike: optimism about the trajectory of the race with cautiousness about all the things that could shift the dynamics.

Obama aides are focused on bolstering get-out-the-vote operations in battleground states to buffer against any late shifts in the race, countering negative ads by a crush of Republican-leaning super political action committees — and living by the mantra of not making too much out of any one poll or event.

Last week, campaign staffers were ordered to be restrained in their response and avoid appearing as though they were declaring victory prematurely when a video surfaced showing Romney telling wealthy donors that 47 percent of Americans believe they are victims.

And, earlier this month, the campaign sought to keep its focus on the economy, the No. 1 issue for voters, rather than get dragged into a foreign policy debate after a U.S. ambassador and three other Americans were killed in Libya.

The worry about next week’s presidential debate — the first of three in as many weeks — is clear at the downtown high rise that houses Obama's campaign inner circle and hundreds of staffers.

On Oct. 3, most Americans will for the first time see the two men standing side by side, a visual that could make Romney seem more presidential to some voters. And the high-stakes showdowns always carry the potential for a slip-up that can be hard to recover from just weeks before Election Day.

Obama has spent the past few weeks preparing for the debates, though advisers won't say much about it, other than to try to lower expectations for Obama and raise them for Romney.

Advisers argue that the debate format — limited to 90 minutes — works against the sometimes long-winded Obama, whom they cast as the underdog on a debate stage.

As Obama adviser Robert Gibbs put it in a CBS interview: “Mitt Romney, I think, has an advantage because he's been through 20 of these debates in the primaries over the last year. He even bragged that he was declared the winner in 16 of those debates.”

For all the concerns, even the most anxious Obama aides take some measure of comfort in an Electoral College map that favors the president. He has more pathways to victory than Romney, whose route to the White House becomes all but impossible without a victory in Florida, where polls show the race is tight, or Ohio, where Obama has pulled ahead in surveys.

Signaling confidence, Obama's team is considering competing in Arizona.

Obama looked at competing in Arizona in 2008 but decided against it because of the support there for home state Sen. John McCain, the GOP nominee. Obama still won 45 percent of the vote.

This year, Obama's team talked early on about running in Arizona, which offers 11 electoral votes, but it never did. Now, with an internal Democratic poll showing Obama narrowly leading Romney, Obama's team might make a play for the state that has seen a 160,000 increase in voter registrations by Democratic-leaning Hispanics over the past four years.

Buying television time in Phoenix, the state's largest city, is expensive and Obama advisers are closely watching their finances.

hat's not to say that competing in Arizona would be all about winning: going up on the air in the state — or sending the president in to campaign there, could force Romney to spend valuable resources defending a state he should be able to count on in the quest to reach 270 Electoral College votes needed for victory.