The presidential debate: A make-or-break moment in U.S. election?

It's uncommon for a sitting U.S. president to hunker down for a full week's worth of election planning in the wooded retreat at Camp David.

But that's where Joe Biden's been.

Then again, there's never been a presidential debate quite like the one he's been preparing for, scheduled for 9 p.m. ET Thursday night.

Two unusually unpopular candidates, Biden and Donald Trump, locked in a close race,will meet in an abnormally early debate, three months sooner than they're typically held.

It is a unique chance for voters to assess them for long, unimpeded stretches Biden requested, and Trump accepted,rules to limit heckling and interruptions.

Trailing slightly in most surveys, Biden appears keen on resetting the trajectory of the race by proposing a debate this early, under this format.

With polls having barely budged a point or two since last year, this is regarded as a rare scheduled event with the potential to shift the campaign.

"The most important 90 minutes of this election season," is how longtime Republican strategist Karl Rove described Thursday'sAtlanta encounter in an op-edin the Wall Street Journal.

"This election may remain close until the end, but if something can put one candidate solidly ahead, it's [this] debate. Get your popcorn. It'll be a heck of a night."

To be clear, debates don't tend to be game-changers.

One reason history recalls the first-ever televised debates between John F. Kennedy and Richard Nixon in 1960 is because they're viewed as an exception.

What debates do and don't do

Kennedy entered those debates polling a touch behind, and exited a touch ahead. His rallies started drawing larger crowds, with previously tentative supporters enthused by his performance. After he squeaked out a narrow election win, he credited the new medium.

"It was TV more than anything else that turned the tide," Kennedy said days after the election.

But even that assertion is in dispute; some researchers question whether that prime-time contrast between a telegenic Kennedy and a pasty-faced, recently hospitalized Nixonhad a statistically meaningful effect at all.

Two researchers who have extensively studied the effect of presidential debates both shared a pair of identical assessments with CBC News about Biden v. Trump.

One: debates tend not to change a race. Two: this time could be different.

We're sailing into unknown seas, they say, because it's so early, because the race has been so static, and because voters have so many doubts about the candidates.

"In 2024, we'll get to study something different with less predictable consequences," John Sides, a political scientist and author at Vanderbilt University, told readers in a recent newsletter.

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He told CBC News that there are conflicting unknowns at play. On one hand, the debate being early might augment its impact; on the other, it could do the opposite, with any post-debate bump potentially dissipating over the summer.

While debates tend to have small, uncertain, effects on polls, saysBen Warnerof the University of Missouri, there are a couple of results he's detected in his years analyzing them. They can, he said, lock in the support of unenthusiastic voters.

This is Biden's chance to convince Democratic-leaners that he's mentally up to the job. AndWarner says Trump can try mollifying Republican-leaners concerned about his criminal cases and his role in the lead-up to the Jan. 6, 2021, attack on the U.S. Capitol.

"This debate has the potential to be among the more influential," said Warner.

But perhaps the most consistent outcome of debates,according to Warner: They improve voters' confidence in their own knowledge of political issues,whichmakes them likelier to vote, donate to campaigns, attend eventsand talk to friends about politics.

About those unique rules

Trump is now grumbling about the rules. There will be no studio audienceand microphones will be cut off when someone's not speaking.

He accepted those rules, designed to avoid a repeat of the sloppy first encounter of 2020. But now Trump suggests he felt pressed; he's made clear in public appearances that he's not thrilled with the no-audience, no-interrupting guidelines.

"It's a very sterile room. Nobody allowed in. And they turn off your mic when you're finished speaking. They're trying to make this very exciting," Trump said, sarcastically, in a recent Michigan speech.

"It's like the mob. They gave me an offer I couldn't accept. They made an offer that was so ridiculous and they knew I was going to say no. And then they could go and they say, 'Biden wanted to debate, but Trump refused to debate. He wouldn't do it.' But I said yes."

Unlike Biden, Trump has not hunkered down for debate prep. He's said to be getting debate briefings from aides, without doing mock debates.

One thing he is doing is trying to rebalance expectations. He has spent years fomenting doubts about Biden's mental acuity, deriding him as a decrepit dodderer.

So what happens if Biden sounds better than expected better than him? Voters will get to judge for themselves as they hear each candidate, without interruption.

Trump is now pre-emptively trying to slather doubt over the validity of a successful Biden evening.

Trump is attacking the CNN debate moderators as biased. More bizarrely, he and his team have insinuated the teetotalling president might be on drugs.

The spin wars

He's doing it repeatedly: suggesting the president might get a shot of performance-enhancing substances, and making wildly false claims about cocaine found last year in the White House.

In Trump's telling, it was hundreds of thousands of dollars' worth of cocaine and: "I think it was Joe['s]." In reality, it was a one-inch baggie, reportedly one-fifth of a gram, worth less than $100, and it was found in a staff entrance the president wouldn't enter.

Trump's motivation is obvious.

These debates still draw large audiences, including 73 million who tuned into the first such contest in 2020. But this represents a much smaller share of voters than the audiences of decades ago.

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Of those tuning in, fewer are undecided voters; most are committed partisans. Among the dwindling number of swing voters in the country, a disproportionate share are less interested in politics, and less likely to be watching.

They'll hear about the debate later, second-hand. And that's where the crafting of narratives is key: Trump is preparing a story for them.

Social media will be a critical battlefield in the post-debate spin wars. Each side will pump out clips meant to reinforce their inevitable claim of victory.

"The voters who are up for grabs here are the least engaged voters," former Barack Obama strategist David Axelrod said on CNN."It's not clear they're going to watch the debate. More likely they're going to get it from social media. So there has to be an awareness of creating moments that can go viral."

In some ways, that speaks to one enduring fact of a presidential debate: their inherent subjectivity. This dates back to those very first TV debates.

Theodore White's Pulitzer Prize-winning book on the 1960 election uses some boxing metaphors in its chapter on the debate, entrenching a habit to be abused by future generations of political writers.

Indeed, Kennedy scored knockouts, delivering uppercuts to Nixon in debates No. 1 and 4. Different polling firms found the young senator was overwhelmingly viewed as the winner in the first and last contest, while the second and third were closer.

But that was among TV viewers. In the polling of those who'd listened to the debates on radio, White wrote, the candidates came off nearly equal.

If those debates happened today, each party would be cherry-picking the most flattering excerpts and pumping them directly into the social media feeds of key voters.

One difference from 1960 is the parties now have Facebook, X,Instagramand TikTokto share carefully selected clips, declare victoryand keep working those referees: the more than100 million Americans who will vote this year.