By KASIE HUNT AND NANCY BENAC, ASSOCIATED PRESS

DENVER — President Barack Obama and Republican Mitt Romney come face to face for the first time in this presidential campaign Wednesday night for a nationally televised debate that will give millions of Americans a chance to size up two fierce competitors in a moment of high-risk theater.

Romney, trailing in polls in a number of key states and running short on time to reverse his fortunes, is angling for a breakout performance in the three 90-minute presidential debates scheduled over the next three weeks.

Obama, well aware that the remaining five weeks of the race still offer enough time for tectonic shifts in his prospects, is determined to avoid any campaign-altering mistakes as he presses his case for a second term.

A pre-debate skirmish Tuesday over Vice President Joe Biden's passing reference to "a middle class that has been buried the last four years" demonstrated how just a few words can mushroom into something larger during a heated contest for the White House.

[HuffPost Live will stream the debate tonight at 9 p.m. ET. Click here for more information.]

Wednesday's faceoff between Obama and Romney on domestic policy at the University of Denver is sure to offer a blend of choreography and spontaneity: Both men have spent hours rehearsing smart lines and pithy comebacks with proxy opponents – yet know to expect the unexpected.

"That's what so tricky about this," says Alan Schroeder, author of a book on presidential debates. "There's never a template for preparing because each one takes its own direction."

The central role of the economy in this election is evident in the topics selected for the first three of the night's six debate segments: The Economy I, The Economy II and The Economy III. The last three segments will focus on health care, the role of government and governing.

Romney has pinned his campaign on the argument that Obama has failed to adequately juice up the U.S. economy, but his challenge is reflected in recent polls showing growing public optimism about the economy and the president's leadership.

Republicans tried to frame the economic debate in their terms Tuesday by pointing to the vice president's comments in North Carolina about the beleaguered middle class as an unwitting acknowledgment that Obama's economic policies have devastated average Americans.

"We agree," GOP vice presidential nominee Paul Ryan declared in Iowa. "That means we need to stop digging by electing Mitt Romney the next president of the United States."

Obama's camp countered that it was the policies of the president's Republican predecessors that had caused the damage.

Biden, at a later campaign event, was careful to say that "the middle class was buried by the policies that Romney and Ryan supported," calling their economic plans an amped-up rework of those from the George W. Bush years.

Romney calls Wednesday's debate the beginning of a monthlong "conversation with the American people," and the debates do tend to consume much of the political oxygen for several crucial weeks.

The candidates will be speaking to a TV audience of tens of millions in one of those rare moments when a critical mass of Americans collectively fix their attention on one event. Fifty-two million people tuned in to the first debate four years ago, and 80 percent of the nation's adults reported watching at least a bit of the debates between Obama and Republican John McCain.

"I think you're going to see the Mitt Romney that really cares about putting Americans back to work," senior adviser Kevin Madden said on "CBS This Morning" Wednesday.

Obama senior campaign adviser Robert Gibbs, appearing on the same program, said "I think what you'll hear the president do tonight is have a conversation with the American people about where we've been over the past four years."

In a quadrennial pre-debate ritual, each campaign has worked overtime to raise expectations for the opponent while lowering the bar for its own candidate. The thinking is that it's better to exceed lukewarm expectations than to fail to perform at an anticipated level of great skill.

But both men are seasoned debaters: Obama has been here before, facing off with McCain in 2008. Romney hasn't gone one-on-one in a presidential debate, but he got plenty of practice thinking on his feet during the 19 multi-candidate debates during the Republican primaries.

On a long day of debate prep – Romney in Denver and Obama in Henderson, Nev., near Las Vegas – both candidates tried to blow off some steam Tuesday. The president made a tourist's visit to nearby Hoover Dam, and Romney fit in a lunchtime outing to a Mexican grill for a burrito bowl.

The two candidates' biggest fans talked up their debating abilities in pre-debate interviews.

Michelle Obama told CNN she's like a nervous parent watching a child performing on the balance beam when her husband debates.

"I do tell him to have fun and relax and just be himself, because the truth is, if he's the Barack Obama the country has come to know and trust, he is going to do a great job," she said.

Ann Romney said her husband always looks around to find her in the debate audience and keeps a paper in front of him that says "Dad" – to remind him to make his father proud.

As for her advice, Mrs. Romney told KMGH-TV in Denver that she tells her husband: "Sweetie, you had five boys. You learned to argue really well and make your points years ago. Just go do that."

Wednesday's format: The moderator, PBS newsman Jim Lehrer, will open each 15-minute segment with a question, and then Obama and Romney will have two minutes apiece to answer. After that, it's up to Lehrer to keep the conversation going and to intervene if one candidate goes on too long.

Obama and Romney have a two-track mission with debate viewers: Motivate core supporters to turn out and vote – at a time when early voting already is under way in many states – and try to lock in some new supporters from among the small subset of viewers who haven't settled on a candidate or whose support for one man or the other is squishy.

The viewers who matter most live in the contested battleground states that will determine which candidate gets to 270 electoral votes on Nov. 6: Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Virginia and, to a lesser extent, Wisconsin.

Recent national polls show the two candidates in a tight race among likely voters. But Obama has the advantage in many of the battlegrounds, including Colorado.

Romney and Obama debate again Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla. Biden and Ryan have their lone debate on Oct. 11 in Danville, Ky.

Obama plans to use the first presidential debate as the hook for fundraisers and recruiting volunteers. Former President Bill Clinton will be in Boston on Wednesday night for Obama, with donors paying $20,000 a person. New York Gov. Andrew Cuomo is headlining a New York fundraiser.

The Obama campaign plans more than 4,000 debate-watching events around the country. And Biden is scheduled to hold a live discussion with supporters that will be streamed online after the debate.

The Romney camp planned 336 debate parties at restaurants, bars, grills, VFW halls and other sites concentrated in battleground states.

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  • 'The Stumble'

    Texas Governor Rick Perry's <a href="http://www.huffingtonpost.com/2011/11/09/rick-pery-forgets-federal-agencies_n_1085312.html" target="_hplink">inability to remember</a> the third agency he would cut as president had many predicting the untimely end of his campaign for president. Perry addressed his mental lapse before reporters after the debate, admitting, "Yeah I stepped in it man. Yeah it was embarrassing. Of course it was."

  • Romney's '$10,000 Bet'

    During a GOP primary debate in late 2011, Romney sought to put an end to then-presidential candidate Rick Perry's insistance that Romneycare was the basis of President Barack Obama's health care reform law. Perry launched in with an attack that he'd repeated before: "I'm just saying, you're for individual mandates, my friend," Perry said. "You've raised that before, Rick, and you're simply wrong," Romney responded, extending his hand toward Perry. "Rick, I'll tell you what: 10,000 bucks? $10,000 bet?" Perry declined, nothing that he wasn't a betting man, leaving Romney to quote a chapter from his book that he cited as proof he had never intended for his health care plan to be used as a national model.

  • Bachmann On Libya, Africa

    At a GOP primary debate in October of 2011, Michele Bachmann (R-Minn.) <a href="http://www.huffingtonpost.com/2011/10/18/michele-bachmann-libya-africa_n_1018814.html" target="_hplink">criticized</a> Obama's foreign policy decisions. "Now with the president, he put us in Libya," she said. "He is now putting us in Africa. We already were stretched too thin, and he put our special operations forces in Africa." Libya is, in fact, a country in Africa.

  • Awkward Silence

    During a 2010 gubernatorial debate, Arizona Governor Jan Brewer <a href="http://www.huffingtonpost.com/2010/09/02/jan-brewer-starts-badly-f_n_703559.html" target="_hplink">struggled to name</a> any of her accomplishments while introducing herself. "We have ... done so much ... We have um, did what was right for Arizona," she squeezed out after a long silent pause.

  • Can't Name Any Supreme Court Cases

    Christine O'Donnell was unable to name a single recent Supreme Court decision she disagreed with, when asked by moderator Nancy Karibjanian during a 2010 Delaware Senate debate. <a href="http://www.huffingtonpost.com/2010/10/13/christine-odonnell-stumped-supreme-court-debate_n_762067.html" target="_hplink">The dialogue</a>: <blockquote><strong>KARIBJANIAN</strong>: What opinions, of late, that have come from our high court, do you most object to? <strong>O'DONNELL</strong>: Oh, gosh. Um, give me a specific one. I'm sorry. <strong>KARIBJANIAN</strong>: Actually, I can't, because I need you to tell me which ones you object to. <strong>O'DONNELL</strong>: Um, I'm very sorry, right off the top of my head, I know that there are a lot, but I'll put it up on my website, I promise you.</blockquote>

  • Can I Call You Joe?

    When Sarah Palin and Joe Biden shook hands at the start of a 2008 vice presidential debate, Palin asked then then-Senator "Hey, Can I call you Joe?" "You can call me Joe," Biden replied. Palin <a href="http://www.politico.com/blogs/bensmith/0110/Two_versions_of_Can_I_call_you_Joe.html" target="_hplink">evidently kept confusing</a> then-Senator Joe Biden's last name with President Barack Obama's, referring to the VP candidate repeatedly as "O'Biden" in debate prep. Her staffers suggested she call him by his first name.

  • Change You Can Xerox

    Hilary Clinton's attempt at a jab toward President Barack Obama got her booed by the audience during a 2008 presidential debate. Clinton accused Obama of plagiarism in his popular speeches, saying "Lifting whole passages from someone else's speeches is not change you can believe in, it's change you can Xerox."

  • 'Likable Enough'

    During a Democratic presidential primary debate in early 2008, then-candidate Hillary Clinton was being pressed on surveys that suggested New Hampshire voters appreciated her resume, but found then-Sen. Barack Obama (D-Ill.) more likable. Clinton appeared to feign insult, drawing sympathetic applause and smiles from the crowd. "Well, that hurts my feelings," she said. "But I'll try to go on. "He's very likable," Clinton continued of Obama. "I agree with that. I don't think I'm that bad." Obama took a brief break from scribbling notes to weigh in. "You're likable enough, Hillary," Obama said tersely, not making eye contact with Clinton. He then returned to his notepad.

  • Al Gore's Sighing

    A 2000 presidential debate seriously hurt Al Gore's campaign when the cutaway shots caught him rolling his eyes and sighing audibly during George W. Bush's answers. Critics say behavior made Gore look elitist and unlikable in contrast with Bush's relaxed and folksy demeanor. Jon Stewart mocks Gore's sighs in The Daily Show clip above.

  • Let Me Finish

    Ross Perot may go down in history for his repeated interruptions of "let me finish" during a 1992 presidential debate. The behavior became fodder for SNL comedian Dana Carvey's Perot impression.

  • Glancing At His Watch

    George H. W. Bush was caught glancing at his watch during a 1992 presidential debate with Bill Clinton and Ross Perot. The now-famous move damaged Bush's campaign, making him look bored and impatient, <a href="http://www.usnews.com/news/politics/articles/2008/01/17/a-damaging-impatience" target="_hplink">reports say</a>. Bush snuck at peek at his watch again during his daughter-in-law Laura Bush's speech at the Republican convention in 2008.

  • Who Am I? Why Am I Here?

    When Independent Presidential candidate Ross Perot picked Vietnam War hero Admiral James Stockdale for his VP nominee, it created a rare three-person Vice Presidential debate in 1992. Stockdale was not a politician and not very well known. Attempting to introduce itself and poke some fun at this, he chose as his opening statement: "Who am I? Why am I here?" Stockdale later said he hoped to follow up the remarks with an explanation of his life, but never got to that point. Instead, the line left viewers wondering the same thing.

  • Dispassionate Death Penalty Response

    When the moderator of a 1988 presidential debate asked Governor Michael Dukakis if he would support the death penalty if his wife, Kitty Dukakis, was raped and murdered, Dukakis dispassionately responded, "No, I don't, Bernard, and I think you know that I've opposed the death penalty during all of my life." He then continued to talk about his stance. Some believe the lack of emotion or passion for the hypothetical situation cost Dukakis the election.

  • You're No Jack Kennedy

    In the 1988 Vice Presidential debate between Democratic VP candidate Senator Lloyd Bentsen and Republican VP candidate Senator Dan Quayle, Quayle was asked if his qualifications were sufficient to inherit the presidency, should it come to that. Quayle responded by comparing his experience level Jack Kennedy's experience level when he sought the presidency. The comparison prompted Bensten to say: "Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy." Quayle responded, "That was really uncalled for Senator."

  • No Soviet Domination

    In the 1976 presidential debate between Jimmy Carter and Gerald Ford, Ford famously stated "There is no Soviet domination of Eastern Europe." The remark came in response to a question about U.S. relations with the Soviet Union, a major concern in the Cold War era, and didn't sit well with an increasingly anti-Soviet public. Ford refused to back down from the claim even after the somewhat baffled debate moderator responded, "I'm sorry, what? ... Did I understand you to say, sir, that the Russians are not using Eastern Europe as their own sphere of influence in occupying most of the countries there and making sure with their troops that it's a communist zone?"

  • Sickly Nixon vs. Fit JFK

    The 1960 presidential debate between John F. Kennedy and Richard Nixon was the first nationally televised debate in the U.S. and <a href="http://www.time.com/time/nation/article/0,8599,2021078,00.html" target="_hplink">is thought to have</a> changed politics forever. The debate was historically declared a win for Kennedy by those who watched it on TV, and a win for Nixon for those who listened to it on the radio. Though the candidates were both strong on the issues, the visibly sweating Nixon looked sickly and pale compared to the young and fit Kennedy.