As speculation mounts over how Trump will go after Harris -- on policy or personality -- Sen. Rick Scott, R-Fla., a top Senate ally, insisted the former president will stick to issues Americans care about.

"He's going to do great," Scott told reporters in the spin room in Philadelphia. "He's going to talk about the issues."

-ABC News' Tal Axelrod

The stakes for this debate are high for both candidates for one very simple reason: The presidential race is extremely close right now. In some of the states most likely to decide the election — such as Pennsylvania, Georgia and North Carolina — 538's polling averages show Harris and Trump within 1 percentage point of each other.

Nationally, Harris leads Trump by an average of almost 3 points, raising the possibility that she could win the popular vote but lose the Electoral College, as Hillary Clinton did in 2016. According to 538's average, Harris's national lead peaked at 3.7 points on Aug. 23, the day after the Democratic National Convention, but her advantage has slipped a bit in the weeks since.

-538's Nathaniel Rakich

Harris and Trump will need to navigate the pitfall-filled debate of their political lives as each tries to persuade millions of voters and viewers that they're the one best suited to be president.

Harris, whose wave of momentum has brought Democrats back to a neck-and-neck presidential race, will have to prosecute the case against Trump while also laying out how her agenda could help the country -- particularly beleaguered middle- and working-class Americans.

Trump, meanwhile, has the task of casting his record on the economy and immigration as superior to Harris' while avoiding distracting personal attacks on Harris.

Republicans and Democrats told ABC News how they should meet the crucial moment. Read more here.

-ABC News' Tal Axelrod

How much difference do presidential debates make? In the past, they've rarely influenced an election's outcome, according to historians who spoke with ABC News.

But the June debate between President Joe Biden and former President Trump was a rare exception, they say, ultimately leading to an unprecedented change in the Democratic presidential ticket.

Now as Vice President Harris prepares to take on Trump in their first presidential debate, experts predict the matchup could potentially produce a similar consequential and history-making moment that could sway undecided voters -- a key voting bloc that could determine who wins the November election.

Read more here.

-ABC News' Ivan Pereira