Hanging over the Republican presidential primary is the doubt that any candidate can overtake Trump. The former president is polling close to 60% in 538’s national polling average -- a historically strong position. In the modern presidential primary era, which dates back to the 1970s, only three non-incumbents before Trump clearly polled north of 50% nationally around this time: Al Gore in the 2000 Democratic contest, George W. Bush in the 2000 GOP primary and Hillary Clinton in the 2016 Democratic race.

Tellingly, all three went on to win their party's nomination -- although it wasn’t always easy.

But Trump is polling in the mid-40s in Iowa and New Hampshire, which might give DeSantis or Haley an opening. Trump still leads in those states by around 25 points, yet we have seen some sizable past leads disappear by the time voting began. In December 2003, Howard Dean held an edge over of Dick Gephardt in Iowa and led John Kerry by 20 to 30 points in most New Hampshire polls of the Democratic primary. But Dean stumbled to a third-place finish in Iowa, and Kerry ended up winning both states en route to the nomination.

In early December 2007, in the Republican primary, Mitt Romney was running close with Mike Huckabee in Iowa and led John McCain by double-digit margins in New Hampshire. But Huckabee won Iowa, McCain won New Hampshire and then also won South Carolina, putting McCain on course to claim the GOP nomination. And while Bernie Sanders didn’t win Iowa in the 2016 Democratic nominating contest against Clinton, he did nearly tie her there in the caucuses after trailing Clinton by 10 to 20 points in December 2015.

It will still be a tall order to catch Trump, however.

Trump is the first former president in more than 75 years to run again after leaving office, and he maintains clear popularity with a significant portion of the GOP. As a result, many of his supporters say they’re only going to support Trump and aren’t considering alternatives, unlike large swaths of voters in past primaries. This raises Trump’s support floor and lowers the support ceiling for candidates like DeSantis or Haley, who likely need to win over at least some Trump backers to have any chance of winning.

-Analysis by Geoffrey Skelley of 538

Haley and Ramaswamy -- who are both set to be on stage again -- have had multiple fiery exchanges so far in the GOP debates, and Wednesday could see more sparring.

Among their mutual jabs, during the second primary debate, in September, Haley said she felt "a little bit dumber" listening to the entrepreneur.

"TikTok is one of the most dangerous social media apps that we could have. I -- honestly, every time I hear you, I feel a little bit dumber for what you say," Haley said at one point, noting Ramaswamy's recent participation on TikTok despite his criticism of the platform.

Ramaswamy responded that the Republican Party would be better served "if we're not sitting here hurling personal insults and actually have a legitimate debate about policy."

Discussions on the social media platform prompted another notable exchange during the third debate, last month. After Ramaswamy mentioned that Haley's daughter has used TikTok, despite criticism of the Chinese-owned app, Haley lashed out.

"Leave my daughter out of your voice," she said, adding with an eye roll: "You're just scum."

Ramaswamy took another swipe at Haley during the third debate, saying, "Do you want a leader from a different generation who's going to put this country first, or do you want Dick Cheney in three-inch heels?"

Haley responded, "I'd first like to say they're five-inch heels, and I don't wear them unless you can run in them. The second thing that I will say is, I wear heels, they're not for a fashion statement -- they're for ammunition."

-ABC News' Meredith Deliso

The debate is the final one scheduled by the Republican National Committee. In less than six weeks’ time, on Jan. 15, Iowa will hold its caucuses and, a week later, New Hampshire voters will go to the polls in their primary -- as the 2024 presidential race gets underway.

Candidates are eyeing these two January contests to get some liftoff with the public before Nevada and South Carolina hold their own contests in February, followed by Super Tuesday on March 5, when more than 10 states hold primary elections.

By then, just three months from now, a clear front-runner for the parties' eventual nominations very well could have emerged.

-ABC News’ Chris Boccia

I’m Katie Sanders, the editor-in-chief of PolitiFact. I’ll be chiming in with fact-checks throughout the debate, but know that I’m one of approximately 20 PolitiFact journalists watching for candidates’ exaggerations and false attacks -- the kind of moments that make you go “really?!” when you hear them.

We’re coming in fresh from covering last Thursday's Gavin Newsom-Ron DeSantis debate on Fox News. For this one, it's totally fine by me if the candidates want to stick to claims we've already covered. But we're ready for surprises.

-Katie Sanders, PolitiFact