WASHINGTON (AP) — As millions of Americans watch, President Barack Obama and challenger Mitt Romney will be chasing opposite goals in their first debate. The key for Obama is to sit on his narrow lead without mishap, while Romney's challenge is to shake up the race and connect with voters.

And while debates are seldom determinative, they can alter the direction or pause the momentum of a presidential contest. Five weeks before the election, the public's attention is becoming more focused, opinions are gelling and in some states votes are already being cast.

First debates have not been kind to incumbent presidents seeking re-election. So to the extent that history holds lessons, they can only be cautionary for Obama and encouraging for Romney when they meet Wednesday night in Denver.

Obama is vulnerable and Romney will seek to wound him. Joblessness stands above 8 percent, the economy is growing at a snail's pace and Obama's health care law remains a contentious topic with voters.

Romney's message: The country can't afford another four years of an Obama administration.

Obama's message: The country would be worse off without his policies and he needs four more years to finish the job.

But there is also urgency for Romney.

"We may have five weeks left to the election, but this thing is going to be over in three weeks. Maybe in two," said Michael Dennehy, a top adviser to John McCain's 2000 and 2008 presidential campaigns. "So the window is closing. Romney needs to take every single opportunity that he has."

A divided electorate, a president with a tailwind despite unpopular policies and a challenger struggling to gain traction as the first debate of the general election looms. It's not the first time Americans have watched this scenario.

In 2004, President George W. Bush was riding a post-convention bounce of 7 to 8 percentage points. Sen. John Kerry of Massachusetts wasn't breaking out with his message of foreign policy ineptitude by the Bush administration. But by the end of their first debate, Kerry was declared the winner and within days he was back in the hunt, tied with Bush in national polls.

The uncanny parallels are not lost on Obama and Romney.

What can Romney do?

To gain attention Romney must methodically define the current weak economy as a failure of Obama's policies. If the Republican convention and Romney's public comments are a guide, he will blame spending, regulatory overload and uncertainty over Obama's health care overhaul as factors that dragged down economic growth.

He will challenge Obama's plan to raise taxes, saying that even if they target wealthy taxpayers they will hurt small businesses and place yet another obstacle in front of economic growth.

Expect him to confront Obama's assertions about the Republican economic plan and challenge the accuracy of Obama's claims.

The debate is divided into six distinct 15-minute segments. The first three are devoted exclusively to the economy and the fourth is about health care. With 50 million or more viewers expected, Romney's greatest leverage comes in those first 45 minutes when the audience is most engaged and when the issue — the economy — is most favorable to him.

Tad Devine has a unique perch to assess this fall's presidential debates. The veteran Democratic operative was a senior campaign aide to Sen. Edward M. Kennedy in 1994 when Kennedy fended off a challenge from Romney for the U.S. Senate. He also was a top aide to Kerry in 2004.

"Demeanor is very important," Devine said. "Kerry was aggressive, but he didn't go outside the frame."

Dennehy conceded that Romney will have difficulty connecting with his audience.

"Facts are facts: Obama looks more real than Romney does," he said. "Romney has to illustrate — to independent voters, particularly — that he can help get us out of this mess."

But even after the debate, and even if he's declared a winner, Romney faces a long challenge — one that Kerry did not overcome in his eventual loss to Bush.

Terry Holt, who was Bush's campaign press secretary in 2004, said the 2004 electorate was far more volatile than it is today and predicted that even if Romney were to win, polls would not show a big swing in his favor. "Whatever change is going to happen in Romney's favor is going to happen in a more steady, harder to discern way," he said.

What can Obama do?

First, be ready. Kerry, who is playing Romney in debate practice rounds with Obama, was not the only challenger to rattle an incumbent in their first debate. Ronald Reagan got the best of President Jimmy Carter in 1980, but then Walter Mondale was perceived as the winner over Reagan in 1984.

Holt said Bush simply didn't prepare well enough for his debate with Kerry.

While Obama needs to exude confidence in his policies, he also has to avoid the trap of smugness. He barely knows Romney and could find that it's easy, with unfamiliarity, to display disdain for his challenger.

He must hit his marks. Eager to lower expectations, Obama aides have cast him as long-winded in his responses. The fact they've drawn attention to that potential weakness means he will be precise in his points and concise in his answers.

Don't look for Obama to lead an attack against Romney, but be prepared for a fierce counter. He will challenge the math of Romney's tax plans and will probably allude to Romney's claim that the 47 percent of Americans who support Obama believe they are victims, entitled to government support.

Romney's experience as founder of the private equity firm Bain Capital or his personal investments in offshore accounts will probably be fodder for Obama's counteroffensive, though the incumbent will be wary not to sully his positive ratings for likability.

Count on Obama to make a case for economic improvement under his watch, a task supported by some economic indicators but still a difficult sell given the nation's high joblessness. Obama will also argue the economy would be worse without his policies, another tricky argument that seeks to prove an unknown.

"The president's challenge is to provide context for the economy," Devine said. "To explain why what he has done has benefited the nation."