The first debate will be a key moment for Republican Donald Trump, who has closed the polling gap in recent weeks against Democrat Hillary Clinton.
"We will make America strong again. We will make America safe again, and we will make America great again! Thank you, God bless you, thank you!"
Clinton looks to the debate to reassure her supporters and to push her message of wanting to build a more inclusive economy.
"We are all of value: in the United States of America, the greatest country in the world, we believe everyone is created equal!"
The debates loom as a potentially game-changing event, says Matthew Dallek.
"Right now the election is closer than a lot of people anticipated, and so it matters a great deal for both of them."
Televised presidential debates began in 1960 with Richard Nixon and John Kennedy. And over time, the debates have had an impact on some presidential elections, says George Mason.
"In a normal year, the debates are one of the only ways to move the needles after the conventions. They are the moment where more people tune in and watch."
Both Trump and Clinton performed well in their respective party primary debates earlier this year. But Trump's unpredictable nature presents a challenge for Clinton, says Stephen Wayne.
"The thing about Donald Trump is that he is not as disciplined as Hillary Clinton. If you get under Donald Trump's skin, he is apt to react in a very personal way."
Both contenders suffer from high disapproval ratings from voters, and the debates represent both opportunity and risk, says Jeremy Mayer.
"What a debate can do is give a candidate a second chance to make a different impression or cement a negative impression."
There will be a total of three presidential debates and one face-off between the two vice-presidential candidates, Democrat Tim Kaine and Republican Mike Pence.
Jim Malone, VOA NEWS, Washington.