WASHINGTON (AFP) - Neither Hillary Clinton nor Donald Trump is the clear favourite as the two major-party candidates for the White House go head-to-head in their crucial first debate Monday (Sept 24).
If Clinton, the Democratic nominee, possesses deep Washington experience and in-depth knowledge of the issues, she lacks what supporters of her Republican rival see as his authenticity.
The two candidates have been laying into each other for a year but have never appeared face-to-face on the same stage, just strides apart.
That surely will be one of the biggest attractions of the highly touted televised encounter, which analysts say could draw as many as 100 million viewers - a number never seen before in American politics.
The debate will be history-making in another way: no woman has ever taken part in a US presidential debate since they began in 1960, with Senator John F. Kennedy squaring off in a Chicago studio against Vice President Richard Nixon.
Most voters have already made their choice ahead of the Nov 8 election, and the series of three presidential debates (the others are Oct 9 and Oct 19) will probably just reinforce them in their decisions.
Still, the debates can have an impact on voters still undecided on who should succeed Barack Obama. And these wavering voters appear to be more numerous than four years ago, amounting to nine percent of the electorate, according to an NBC poll.
What do the candidates need to do to win them over?
"We typically don't tune in to our televised debates to see who's the smartest of the two candidates and which one has the most facts and figures and policy information that they're spewing for 90 minutes," Mitchell McKinney, professor of political communication at the University of Missouri, told AFP.
McKinney, a specialist in political debates, said television viewers favour candidates who manage to communicate their vision in a few simple, compelling and memorable phrases.
Clinton, with her diligent and detailed mastery of the issues, will have to avoid falling into the trap of giving overly technical and exhaustive responses to the moderator's questions.
"You need to generate a more emotional connection with voters if you're going to prevail," said communications consultant Carmine Gallo.
In the words of Obama, who was asked what debate advice he would offer his former secretary of state: "Be yourself and explain what motivates you." That has been a constant challenge for Clinton, the least loved of Democratic presidential candidates in years, according to polls.
Clinton herself acknowledges that she cannot match the charisma of her husband, Bill Clinton, or of Obama. More than half of Americans say they are not sure they can trust her.
During her first run for the presidency, in 2008, Clinton presented herself as a tough, Thatcher-style "iron lady."
This time, she stresses her role as a pioneer for women's rights and polishes her image as a grandmother, in an effort to seem more likable and accessible.
But it will not be easy for her to wipe away, in a 90-minute debate, an image forged in public opinion over a quarter-century.
Her strength may lie in her ability to counter any attack with an effective verbal comeback.
"What are those one, two or three key messages that they want people to share on Twitter and social media?" Gallo asked. "Listen for the sentence or two that she repeats several times in the conversation." "Trump connects to his voters on a deeply emotional level, and that can be quite difficult for an opposing candidate to match, because emotion often trumps data," Gallo said.
In this area, the wealthy populist, former host of a successful television program, enjoys a clear advantage.
No candidate in this campaign - with the possible exception of Democratic senator Bernie Sanders - has matched Trump's ability to electrify crowds of thousands.
But Trump did not hold the upper hand in every one of the 12 Republican primary debates. He often stood aside to let the other candidates rip into one another.
In the later debates, when only a few of his adversaries remained, he often resorted to disruptive tactics, using scathing phrases or demeaning nicknames to savage his opponents.
"Unlike the primary debates, where there were multiple candidates on the stage and therefore we heard from Trump periodically, in a 90-minute debate where he's going to have half the time, he can't fill all of this time with one-liners, with self-praise, with the glib attacks - that will wear thin," McKinney said.
"He will have more opportunities to provide substance. When that time comes, will he have the substance? We will be watching to see."
The concern of the Clinton team is that the moderator, Lester Holt of NBC, will toss simpler "softball" questions in Trump's direction while pressing Clinton with a much more challenging interrogation.
Either way, these exchanges are sure to be intensively analysed afterward as part of the continuing debate... on the debate.