A hallmark of the American electoral process, debates are considered a key showcase for candidates seeking to hold the nation's top office.

Memorable moments include Ronald Reagan’s witty response to questions about his age and Kamala Harris’ challenge to Joe Biden on segregation and race, which prompted the viral line: "That little girl was me."

But not all debate highlights are positive. Richard Nixon’s sweating, Al Gore's relentless sighing and Rick Perry’s "oops" gaffe were considered costly to their campaigns.

"These are the only times we see them side by side," one expert told ABC News. "They provide opportunities for candidates to either help themselves or hurt themselves."

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-ABC News' Alexandra Hutzler

This debate might be going out to a national audience, but the 2024 Republican presidential primary will be decided on a state-by-state basis — and because they’re the first to vote, Iowa, New Hampshire, South Carolina and Nevada will have an outsized impact on the race.

We’ve been collecting data on how many times each candidate has visited each early state, and there are some revealing trends. For one thing, the candidates seem to think Iowa and New Hampshire are much more important than South Carolina and Nevada. They’ve visited the first two more than 90 times each but South Carolina only 30 times and Nevada only four times. In addition, some candidates seem to be putting all their eggs in either New Hampshire’s or Iowa’s basket. For example, Christie and Hurd are focusing almost exclusively on New Hampshire — perhaps the famously independent Granite State will prove more receptive to their anti-Trump brand. Hutchinson, meanwhile, is focusing on Iowa.

Overall, Ramaswamy and Haley have paid the most visits to early states — though that’s partly because they announced their campaigns earlier than most other candidates.

-Analysis by Nathaniel Rakich of FiveThirtyEight

Trump maintains a lead of 28 percentage points over his nearest competitor in South Carolina, according to the latest polls.

Haley and Scott, the two candidates from South Carolina, are both polling higher in South Carolina than they are nationally, according to our polling average. Trump is polling about 7 points lower in South Carolina than in national polls.

Only one pollster, the Manhattan Institute has tested the favorability of Republican candidates in South Carolina since July 1. That survey found Scott and DeSantis had the highest net favorability in the state (+56 and +55 percentage points, respectively), and Trump and Haley close behind (+39 and +33 points, respectively). Pence had a net favorability of +1 point, and Christie was underwater by -36 points.

Economic issues are most important to Republican primary voters in South Carolina, according to a July survey from Fox Business/Beacon Research/Shaw & Co. Research. Fifty-one percent of Republican voters said economic issues would be most important for deciding their vote. Twelve percent each said immigration issues, social issues and foreign policy were most important.

In an August survey by National Public Affairs, 40 percent of South Carolina Republican voters said that Trump wouldn’t lose their support even if he did not show up to the debate. Twelve percent said that they support Trump, but if he didn’t debate they would be open to or definitely supporting someone else, 32 percent said they wouldn’t vote for Trump regardless, 9 percent said they probably wouldn’t watch the debate anyway and 6 percent were unsure.

-Analysis by FiveThirtyEight

Trump maintains a lead of 30 percentage points over his nearest competitor in New Hampshire, according to the latest polls.

Christie and Scott are both polling about 5 points higher in New Hampshire than they are nationally, according to our polling average. Trump is polling about 10 points lower in New Hampshire than in national polls.

In a July survey from the University of New Hampshire, only 36 percent of New Hampshire Republican voters said they have definitely decided who they will vote for. Forty-five percent said they are leaning to someone, and 17 percent said they are still trying to decide. Voters who said they plan to vote for Trump are most likely to say they are definitely decided (76 percent), whereas voters who said they plan to vote for DeSantis are most likely to say they are leaning to someone but not definitely decided (72 percent). Fifty-three percent of voters who said they plan to vote for Christie said they are still trying to decide who to vote for.

Scott and Ramaswamy have the highest favorability among New Hampshire Republicans. Pence, Hutchinson and Christie are all underwater.

New Hampshire Republicans are more interested in economic issues than social issues, according to a late June poll by Saint Anselm College. Seventy-five percent of registered Republican primary voters said they would prefer the 2024 election be dominated by debate over economic issues, while just 15 percent said they would prefer social issues.

-Analysis by FiveThirtyEight