Will the tables turn on Trump's age and mental fitness?
In the most recent ABC News/Ipsos poll, Harris had an advantage over Trump when it came to who Americans think has better "physical health" and "mental sharpness" to serve effectively as president.
Trump has recently gotten attention for his rambling and often-incoherent speaking style, and while that's nothing new for him, this will be his first time directly facing a new opponent, who's nearly 20 years younger than him. Whether or not Trump's age or mental acuity come up explicitly in tonight's debate, how he handles himself — and how viewers respond afterward — could help him put some of those concerns to rest, or exacerbate them and give Harris an opening.
—538's Tia Yang