United States President Barack Obama and Republican challenger Mitt Romney will be chasing opposite goals in their first debate with millions of Americans watching. 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 on Wednesday night in Denver.
The rivals will step up to podiums at the University of Denver in the Rocky Mountain state of Colorado, at 7:00 pm (0100 GMT) to clash over the economy and other domestic issues. Five things to watch
But veteran anchor Jim Lehrer, who will steer the debate for tens of millions of viewers at home, has leeway under rules thrashed out by the two campaigns to bring up other burning issues.
Obama is vulnerable and Romney will seek to wound him. Joblessness stands above 8%, 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. Key details of the 2012 US presidential debates
"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.
YouTube, Xbox to stream US presidential debates
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. Romney attacks Obama on West Asia
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 2012 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 US 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." Issues at stake in US election
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 electorate that year 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.
Challenges for Romney
Republican candidate Mitt Romney, 65, a multi-millionaire former venture capitalist, could come under scrutiny over his complex offshore tax arrangements, which Democrats have highlighted to press the case that he is indifferent to middle class struggles.
Romney is under pressure to produce a strong performance on Wednesday at his first face-to-face debate with President Barack Obama to try to turn around a race for the White House that has been edging away from him.
Running behind in the polls, Romney is more in need of a victory than Obama at the University of Denver debate, the first of three such face-offs scheduled in the next four weeks.
"I think he's got to have a pretty convincing win," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University. "He's had a bad few weeks and he needs to change the narrative of the campaign."
It was only one of several recent stumbles by the former Massachusetts governor in his second presidential bid. Behind closed doors, Romney gossips, imitates Kissinger
At the Denver debate, Romney needs not only to repair some of the damage from the video. He must raise questions about Obama's handling of the US economy and explain how his own plan would create more jobs and cut the budget deficit.
Romney must get through the debate without losing his cool and without appearing to be disrespectful to Obama, who many Americans like personally despite his struggle to create jobs.
And the often robotic Republican could do with showing some personality to make voters feel more comfortable with him.
"Americans who are thinking about voting for Romney need to hear from him about how he would change the country for the better," said Republican strategist Ron Bonjean.
"They're leaning toward the devil they know, which is President Obama. Romney has to knock it out of the park by showing the contrast between himself and Obama."
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. Medicare Study
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 likely allude to Romney's claim that the 47% of Americans who pay no federal income taxes support Obama because they believe they are victims and entitled to government support. Obama offers more of hope
Romney's experience as founder of the private equity firm Bain Capital or his personal investments in offshore accounts will likely be fodder for Obama's counteroffensive, but the president must 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."
Challenges for Obama
President Barack Obama, 51, could face a grilling on his administration's shifting account of the attack on the US consulate in Benghazi, Libya on September 11. Can Obama bridge rift with Muslim world?
The Democrat has the challenge of answering why Americans should consider themselves better off now than they were four years ago, a key measure in every presidential election. He needs to explain what he would do to rekindle job creation in a second term. Romney slams Obama on foreign, military policies
The US jobless rate has been above 8% for 43 straight months and is the top priority of voters. The Obama camp argues he inherited a tough economy from Republican predecessor George W Bush.
Many voters seem willing to cede him that point but nonetheless are looking for a clear way out of the economic doldrums.
"He's got to reassure people who like him that it's OK to vote for him again," said Yepsen.
"I think Americans like the man; they're a little bit concerned about the job he's done. And he's got to bring them back home."
So far, Obama has offered little in the way of a second-term governing agenda beyond more of the same policies, amid rising debt, budget deficits and increasingly expensive entitlement programs. His first term has been marked by fierce partisan battles that have frozen Washington into political gridlock.
Obama's campaign has cast Romney as a wealthy elitist who is out of touch with the plight of everyday Americans. Who's ahead, what are the issues
Body language will be closely watched to see if Obama can fight a tendency to be condescending and professorial and whether Romney can resist arguing about the debate rules or who gets the most time to speak, as he did during Republican primary debates.
The Obama camp seized on a New York Times article that said Romney had benefited financially on his offshore holdings. The Obama campaign charged that Romney had "failed to come clean with the American people."
A comment from vice president Joe Biden gave the Romney campaign an opportunity. Biden accused Romney of seeking to raise taxes on America's middle class, which he said "has been buried the last four years."
"Of course the middle class has been buried," said Romney's vice presidential running mate, Paul Ryan.
"They're being buried by regulations; they're being buried by taxes; they're being buried by borrowing. They're being buried by the Obama administration's economic failures."
Experts are not necessarily in agreement on whether debates can serve as a turning point in a presidential election.
But history shows there are plenty of cases where they have cast some candidates in a negative light, from Al Gore's heavy sighs and eye-rolling during a 2000 debate with George W Bush to Richard Nixon's profuse sweating during his encounters with John F Kennedy in 1960.
(With inputs from AP, Reuters and AFP)
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