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

The moderators finally turn to discussing Trump, who has skipped the debate for a (pre-recorded!) interview with Tucker Carlson. It's clearly intentional that they waited this long — Trump is ahead by 37 points in our national polling average, so he's an obvious elephant (not) in the room. In some ways the entire debate is moot because of that lead. Remember that these candidates are running for second and third place right now, not first.

-Analysis by G Elliott Morris of FiveThirtyEight