Barack Obama and his Republican foe Mitt Romney clash Wednesday in the first of a trio of debates, a test of nerve, temperament and presidential mettle before tens of millions of viewers.
In these Sept. 26, 2012 file photos, President Barack Obama and Republican presidential candidate Mitt Romney both campaign in the battleground state of Ohio.
The televised debates, which have the feel of a heavyweight title fight as a feisty challenger seeks to knock out or outpoint the current champion, are the last best hope for Romney to save a sagging campaign.
And they represent a potential minefield for President Obama as he seeks to cling on to a narrow lead in his quest to win a second term on November 6.
The debates, a regular feature of presidential campaigns since a stubbly Richard Nixon lost to a youthful John F. Kennedy in 1960, allow a chance to measure up the men who would be president as they stand side-by-side.
Challengers like Romney must first convince voters they have the intellect and disposition needed of a president in a dangerous world, and showcase a personality that will not grate over four years in the White House.
Incumbents have an advantage because by definition, they are already viewed as "presidential" but as ex-leaders like Gerald Ford and George H.W. Bush found out, a slip can seriously damage a campaign.
Romney's biggest test on Thursday may be to wrestle with a president who polls show is liked and trusted by a majority of voters, without seeming disrespectful or dismissive.
He endured a seemingly endless string of debates during the Republican primary campaign, but events with multiple candidates are a far cry from the concentrated pressure of a one-on-one face-off.
The more likeable Romney has tried to be throughout the campaign, the more unlikeable he has seemed to become, as he has struggled to show a personality that those close to him say is warm, caring and humorous.
The Republican has also left a trail of gaffes so Obama is likely to try to goad him in the hope of drawing out more unfortunate off-the-cuff comments.
"Romney has to go in there with a lot of preparation, with a very tight script, with a homebase in terms of message, that is a place he's always going to return to," said veteran Democratic political consultant Bob Shrum.
"And then he has to memorize and stick to that script because he can't be trusted when he's spontaneous."
For Obama, the biggest challenge may be that he is out of practice over the four years since he out-debated his last Republican opponent, John McCain.
Presidents are habitually treated with deference and are not used to being cross-examined in public, so Obama must guard against coming across as entitled or irked by Romney's barbs.
On Spanish language television channel Telemundo last month, Obama appeared aggravated by blunt questions and flat footed in some answers, suggesting that the debate practice he is getting at a Nevada resort is much needed.
While the president is famous for a flashing smile, he can be a cold fish as well, and some who meet him interpret his sometimes impersonal manner as arrogance and aloofness.
In a Democratic primary debate with Hillary Clinton, one comment to her -- "you're likeable enough" -- came across as prickly and condescending.
But the president's cool demeanor and more winning personality traits were on show when he met McCain, and his steely response to the unfolding financial crisis was key to him winning the presidency.
McCain by contrast, was irascible and displayed apparent disgust for Obama, appearing unwilling to look him in the eye.
Obama has made it known that he understands his professorial answers are sometimes a turn off, and he has been honing more manageable soundbites.
Wednesday's debate is on the economy and domestic policy, but Jim Lehrer, a PBS television anchor moderating his 11th presidential showdown, has latitude within the rules to bring up other subjects.
So Obama could be asked about his administration's shifting narrative on the deadly assault on the US consulate in Benghazi on September 11, which Romney says is proof that his foreign policy is falling apart.
Both campaigns play the often absurd game of setting low expectations.
"Governor Romney, he's a good debater... I'm just okay," Obama said, playing down his reputation as a master orator in the hope of being set a lower bar for success.
But the Romney campaign's careful expectations-setting took a blast of friendly fire from Republican New Jersey governor Chris Christie.
"This whole race is going to be turned upside down come Thursday morning," Christie said, predicting an outright Romney victory.
Romney advisor Ed Gillespie indicated Monday that his candidate would make the debate a referendum on Obama's tenure and economic record.
There is no hard and fast rule that debates change campaigns, but sometimes they do -- for example in a single debate in 1980, Ronald Reagan met the presidential test and went on to beat Jimmy Carter.
Kennedy saw his win over Nixon as crucial to him winning the White House.
And Al Gore's huffy sighs and overbearing character dented his campaign in 2000, even though on substance he won the debates over the eventual election victor George W. Bush.
Bush was scored the loser of all three debates to John Kerry in 2004, but still went on to win re-election.