WASHINGTON (Reuters) - If history is a guide, Democrat Barack Obama will have a tough time in the first presidential debate on Wednesday, Republican Mitt Romney will be particularly aggressive, and both will risk committing a damaging gaffe if they wander off their talking points.
The 90-minute showdown in Denver - the first of three televised Obama-Romney encounters in October that will set the tone for the final month of the presidential campaign - will feature two experienced and competent debaters who are at their best in scripted settings.
Neither the Democratic president nor his Republican rival has ever seemed to enjoy the more freewheeling aspects of a candidates' debate, and both have said things during debates that became headaches for their campaigns.
Obama, known for soaring rhetoric and inspirational generalities in his speeches, suffered in some 2008 Democratic primary debates from a diffident and nuanced style that could make him seem flat and uncertain. His most pointed responses often seemed condescending or flippant.
Romney, who survived a grueling series of debates in this year's Republican race, often displayed an efficient and feisty style in rolling over his challengers.
But Romney also can seem stiff and awkward when challenged.
When frustrated, he can also make mistakes - like his offer to bet Texas Governor Rick Perry $10,000 over a disputed point during one Republican primary debate, a scene that was a reminder of critics' charges that Romney is an out-of-touch rich guy.
"They are pretty evenly matched as debaters," said Alan Schroeder of Northeastern University in Boston, who has written a history of presidential debates. "They both tend to be more intellectual than emotional, and they are both articulate and comfortable on camera. But they would both prefer to be in a more controlled setting."
The high-stakes debates - particularly the one on Wednesday, which will be moderated by PBS' Jim Lehrer and focus on domestic policy - could be the last chance for Romney to reclaim momentum from Obama.
The president leads his Republican rival in most national polls and in the politically divided "swing" states likely to decide the November 6 election.
Romney has taken frequent breaks from the campaign trail to practice for the debates, which could draw a television audience dwarfing the 30 million who watched his speech on the last night of the Republican National Convention last month.
The first of the three debates in the 2008 presidential race between Obama and Republican John McCain drew 52 million television viewers.
Debates rarely make a big difference in the final results of presidential races, but a strong performance can give a candidate a bump of a few percentage points in polls, said Mitchell McKinney, a political communications specialist at the University of Missouri.
"You don't normally see game-changer type moments in a presidential debate," he said. "But now the game is almost over, and this is Romney's chance. Can he take advantage?"
Both candidates are proven debaters, but have weaknesses that can be exploited, analysts said.
"If you are Romney, you want to figure out a way to get under Obama's skin, you want to see him get prickly," said Dan Schnur, an aide to McCain's 2000 presidential campaign. "If you're Obama, you want to be a little unpredictable and throw Romney off his script."
Recent presidential debates suggest the first debate could be Obama's toughest.
Three of the last four incumbent presidents to seek a second four-year term - Ronald Reagan in 1984, George H.W. Bush in 1992 and his son George W. Bush in 2004 - suffered difficult first debates, although Reagan and the younger Bush won re-election.
The exception was Bill Clinton, a natural debater, in 1996. Clinton had defeated the elder Bush in 1992, and was a much sharper communicator than Kansas Senator Bob Dole, Clinton's Republican opponent in 1996.
"An incumbent president is used to having his way and being deferred to. He is not as used to being challenged," McKinney said. As a result, "they may come across as perturbed. Their job is to defend their record without becoming defensive."
That could prove tricky for Obama, who also will have to walk a fine line in acknowledging the nation's high unemployment rate and the economic difficulties faced by most Americans, while offering a more hopeful vision for the future.
During the 2008 debates, Obama sometimes sounded like the constitutional law professor he once was, exploring every angle of an issue in a rambling style. His staff says it is trying to curb those tendencies in debate preparations.
Obama was reminded of the danger of straying off message earlier this year when he punctuated a speech about the government help that business owners receive with the line, "You didn't build that."
Romney's Republicans called it an insult to business owners and made "We Did Build That" a theme not only of campaign commercials but the Republican convention.
Obama's most memorable 2008 debate gaffe came in what was widely viewed as an insensitive putdown of Democratic rival Hillary Clinton after a moderator questioned her likability.
"You're likable enough, Hillary," said Obama, who after his election appointed her as secretary of state.
Romney faces his own challenges on likability, and the trick for the former private equity executive will be to mount an aggressive attack on Obama without coming off as too cold, calculating or desperate.
"His problem is that people don't believe what he says," said Democrat Doug Hattaway, an aide in the presidential campaigns of Al Gore in 2000 and Hillary Clinton in 2008. "He delivers the lines fine, but the punches don't land the way they would if he was trusted more, so he comes off as more scripted."
During the primary debates, his offer of a $10,000 bet to Perry was not Romney's only out-of-touch moment.
During a debate in Nevada, he countered an attack on his use of a lawn service that employed illegal immigrants by saying that he had told the company, "I'm running for office, for Pete's sake, I can't have illegals."
On the campaign trail, Romney has unintentionally reminded voters of his wealth and privilege with comments about his friendship with NASCAR owners and his wife Ann's Cadillacs.
In what could be a turning point in the campaign, Romney was secretly recorded at a private fundraiser telling wealthy donors that the "47 percent" of Americans who do not pay taxes are "victims" who depend on the government for handouts and probably would never vote for him.
"Romney has shown a greater facility for gaffes and missteps, obviously," McKinney said.
Even so, Romney - who participated in more than 20 debates in the 2012 primaries and more than 15 during his unsuccessful run for the White House in 2008 - will be the most experienced presidential campaign debater ever to appear in a general election debate, McKinney said.
Romney will have far more camera time and a much bigger audience in a one-on-one debate with Obama than he did during the crowded primary debates. Analysts said he would need to articulate a sharper vision for the nation and avoid looking like he is merely trying to score debate points at every turn.
Gore learned that lesson the hard way in 2000, when he seemed to win the verbal jousting with George W. Bush but turned off viewers with his impatient demeanor and exasperated sighs when Bush was talking.
Editing by David Lindsey and Peter Cooney