Attendees seen greeting supporters outside the debate hall in Milwaukee have included former Arizona gubernatorial candidate Kari Lake, Georgia Gov. Brian Kemp and Trump's son Donald Trump Jr.

While Ramaswamy is still "unlikely" to visit the spin room following the debate, his wife, Apoorva Ramaswamy, his communications director and senior adviser Tricia McLaughlin, two other team members and an undisclosed "special guest" will be there on his behalf, according to McLaughlin.

The spin room is the space where campaigns highlight their candidate and attack their opponents after the debate. Many candidates themselves head there after they take the stage.

-ABC News' Kendall Ross

In anticipation of tonight’s debate, we partnered with Ipsos and The Washington Post to ask likely Republican primary voters what they think about tonight’s debate participants and the candidate field in general.

The 4,968 likely Republican primary voters we polled using Ipsos’s KnowledgePanel expect Ramaswamy’s performance to be best among the eight expected debate participants. According to a five-point scale that we calculated using respondents’ answers (with 1 being “terrible” and 5 being “excellent”), likely voters expected Ramaswamy to perform “very good” (3.59 out of 5). They thought DeSantis would perform second-best (3.51 out of 5) and Scott third (3.43 out of 5).

When it comes to whom respondents think they’ll place their votes for in the primaries, 65 percent are considering Trump and 52 percent are considering DeSantis. About 1 in 4 likely Republican primary voters are considering voting for Pence, Scott and Haley. The remaining six candidates we asked about fell below 25 percent.

We’ll be watching to see how the candidates fare tonight, and if debate watchers’ opinions on whom they’re considering voting for will change after tonight’s performances.

-Analysis by Holly Fuong of FiveThirtyEight

Before tonight’s debate kicks off, FiveThirtyEight, The Washington Post and Ipsos partnered to ask Republican voters about how they might vote in the upcoming Republican presidential primaries. Specifically, we asked 4,968 likely Republican voters which issues are most important to them when deciding which candidate to vote for in the primaries, using Ipsos’s KnowledgePanel.

We offered respondents a list of 16 issues and allowed them to select up to three. Of the topics that we asked about, voters were most concerned about “getting inflation or increasing costs under control” (53 percent of respondents selected this issue), something Republican voters have been concerned about since at least the 2022 midterms, according to a previous FiveThirtyEight/Ipsos poll. Other issues that were top of mind for voters were “controlling immigration” (36 percent), “someone fighting against liberalism and the woke agenda” (25 percent) and “ability to beat Joe Biden” (25 percent).

If tonight’s debate touches upon those top issues, voters might get some clarity on who they are considering voting for when primary season comes around.

-Analysis by Holly Fuong of FiveThirtyEight

As Republican candidates gear up for the debate, many have specific rituals they’ll be doing in preparation.

A campaign adviser for Haley said she will be listening to music, “lots and lots of music: Joan Jett, Queen, Def Leppard and [the] Go-Gos, to name a few.”

DeSantis has spent today with his wife, Casey, and their three children, though a campaign spokesman offered few details about how the family has spent their time.

Hutchinson posted a photo of himself preparing with notepad and laptop in hand.

“Some last minute studying and prep for tomorrow! It’s important to show up and be ready to answer the tough questions, you deserve nothing less than a leader ready to lead on day one,” Hutchinson captioned the photo online.

Pence will plan to spend time in prayer before the debate, while Scott took a tour of the debate stage, which he posted a video of, along with encouragements from his family, to X, formerly known as Twitter.

Others prepared physically, earlier this week.

Ramaswamy posted videos to X of himself playing tennis and him and his wife doing floor exercises in a gym throughout the lead-up week.

Burgum was playing a stress-relieving pickup game of basketball on the eve of the debate, according to a campaign press secretary, when he sustained an Achilles injury that called into question his ability to participate in the event.

He will be participating, albeit on crutches, per a photo posted to his X account.

-ABC News’ Gabriella Abdul-Hakim, Libby Cathey, Abby Cruz, Will McDuffie and Olivia Osteen