By Gregory Krieg, Eric Bradner, Steve Contorno and Daniel Strauss, CNN

(CNN) — The final Republican primary debate before the Iowa caucuses will be a one-on-one showdown between former South Carolina Gov. Nikki Haley, now riding a wave of momentum in the polls and boosted by a barrage of outside spending, and Florida Gov. Ron DeSantis, whose performance in the Hawkeye State could make or break his campaign.

Former President Donald Trump will again be elsewhere, making a solo appearance on Fox News rather than joining his rivals on stage at Drake University in Des Moines.

The debate, scheduled for 9 p.m. ET and hosted by CNN, will provide Haley and DeSantis one final chance to pitch themselves – and make the case against one another and Trump – before the first votes are cast next week. Entrepreneur Vivek Ramaswamy and New Jersey Gov. Chris Christie failed to make the stage under the heightened qualification threshold, a reflection of the increasingly broad margins separating the contenders from what began as a field of nearly 20 candidates.

What remains the same, however, is Trump’s absence. He has skipped every GOP primary debate to date and will do so again a day after opting to appear in the DC Circuit Court of Appeals for oral arguments over whether he should have immunity against federal charges of attempting to subvert the results of 2020 presidential election.

Recent polling on that question revealed a predictable split, with 86% of Democrats and about two-thirds of independents saying Trump should not have immunity. Nearly 7 in 10 Republicans said he should 69%.

Whether and how Haley and DeSantis go after Trump is, as it has been through months of debates, the defining question entering this debate. Here is more on that – and what else to watch for:

Taking on Trump – or each other?

The head-to-head debate could help Republican primary voters and donors who are interested in moving on from Trump settle monthslong questions about which candidate is best positioned to seriously challenge the former president.

But the two share a critical interest: Proving that the undercard race even matters, because at least something approaching half of the GOP electorate is willing to move on from Trump.

The Iowa caucuses, in less than a week, will be the first test of whether the rest of the field combined can keep the former president under the 50% mark — and could demonstrate that there’s a path to victory even if the race is narrowed to Trump and whoever proves to be his strongest rival.

Haley and DeSantis have both sharpened their attacks on Trump in recent weeks. The Florida governor has argued that Trump failed to deliver on his 2016 campaign promises. Haley said in a CNN town hall last week that “chaos follows” the former president. And while both have largely given Trump a pass on the specifics of his legal battles over his role in attempting to overturn the 2020 election, they are increasingly making the case that his seemingly never-ending schedule of court appearances and looming trials in the coming year would damage him in the general election.

Will Haley and DeSantis have their eyes on the long game, and team up against Trump? Or will they focus on their immediate futures, and battle for what polls suggest could be a distant second place finish in Iowa next Monday? The answer might not just shape the 2024 primary race, but offer windows into whether either or both candidates have eyes on their own political futures in a post-Trump landscape, as well.

The man who wasn’t there

It’s one thing to take a pass on the early debates, when a frontrunner has little to gain, but it’s quite another to turn down an opportunity to stand across from your rivals days before voters go to the polls.

Though Trump will be in Iowa enjoying a far friendlier audience on Fox, he is missing out on a last chance to impress caucus-goers by facing off with his opponents – or at least mock, harry and undermine them in a forum that he dominated during his run to the 2016 GOP nomination.

If Trump and his team have gotten it wrong, it will not take long to find out.

Anything less than a clear victory for the former president next week has the potential to upend a race that has been mostly stagnant, at least at the top of the heap, for more than a year. Trump and his team don’t want to give his rivals a thread to hang onto, part of the reason they’ve flooded the state over the past couple weeks.

But there is precedent here and, for Trump, it’s another reminder of eight years ago when he entered the caucuses as the leader in the polls but left for New Hampshire finishing a disappointing second to Texas Sen. Ted Cruz.

DeSantis’ prep

DeSantis, eager to appear willing to take on all comers, has repeatedly challenged Trump to a debate (to no avail) and has even sparred with someone not in the race, Democratic California Gov. Gavin Newsom, amid his flailing efforts to move the needle in a race seemingly stuck for months.

As it is, he has long anticipated a one-on-one debate with Haley as a last-ditch chance to provide a spark for his campaign before the Iowa caucuses – or extinguish hers. He quickly announced his acceptance of the CNN debate invitation and his campaign publicly badgered Haley until she did the same. While his campaign would not say whether he would target Haley or Trump more, a senior campaign official hinted that Haley “won’t be able to hide” on Wednesday night.

DeSantis aides believe his time working with famed veteran debate whisperer Brett O’Donnell has paid off and the preparation sessions – some held in Iowa, others in Tallahassee – have helped to minimize his uncomfortableness in such settings. It was during one of these sessions that his debate team workshopped DeSantis’ response when moderators in the first debate asked all the candidates to raise their hand if they believed climate change was real.

“We’re not schoolchildren. Let’s have a debate,” DeSantis said, derailing the exercise.

His debate prep, which once included sessions with or without Trump, are now absent such contingencies. Instead, he is squarely focused on Haley, who is played in these sessions by Jessica Bartlett, a top lawyer for the campaign, according to a person with knowledge of the debate preparations. His campaign declined to comment on details of his debate strategy or preparations.

Bartlett, a longtime GOP operative, has meticulously studied tape of Haley’s remarks and past debate performances going back to her time in South Carolina. During the actual debate, she regularly predicts the former governor’s responses before she says them, the person said.

DeSantis’ attention to these sessions has been serious – a sign of how much his campaign has priorities these high-stakes showdowns. Sometimes he will pause and question whether Haley would “really say that” in response to a particular attack based on his knowledge of her past remarks, the source said. At other junctures, he and Bartlett practice talking over each other until one side gives up.

Haley’s aides, in the lead-up to the debate, have played it characteristically close to the vest, refusing to divulge many details about the former United Nations ambassador’s strategy.

Setting expectations for Iowa

The Iowa caucuses traditionally cull both parties’ presidential fields — often significantly reducing the number of candidates. Though DeSantis and Haley might both hope for a stunning upset next week, one major task between now and then is managing expectations — proving to their donors and supporters that even without a win, they have paths forward in the GOP race.

DeSantis on Sunday said at an Iowa rally he plans to stay in the primary for the “long haul” regardless of the caucus results.

“It’s a long process. It’s an arduous process. I think Iowa is going to be a great way for us to start the process, but we certainly have a lot of road after that. And, you know, we’re girded for that long battle,” he said.

Haley, meanwhile, is polling in a strong second place in New Hampshire and South Carolina — states that have long been more critical to her 2024 hopes than Iowa. However, she risked backlash in the Hawkeye State when she joked last week in New Hampshire that its primary would “correct” Iowa.

Still, a second-place finish ahead of DeSantis, as the race shifts to states where polls show she is well ahead of the Florida governor, would be significant.

Mark Harris, the lead strategist for the pro-Haley super PAC SFA Fund Inc., told reporters last week that he sees Iowa as a battle for second place between DeSantis and Haley.

“Right now, I think it’s a tough fight for second between us [Haley] and him [DeSantis]. You know, I think it’s very possible we get there. We’re working on it. I’m encouraged by the direction we’re headed,” Harris said.

Haley’s Civil War problem

Haley faced days of criticism for failing to name slavery immediately when she was asked at a recent New Hampshire town hall to explain the cause of the Civil War.

Though Haley quickly sought to clarify her remarks, the episode marked the first time — after months of climbing in the polls — she’d faced serious backlash over a stumble of her own making.

President Joe Biden took a jab at Haley on Monday during a speech at the Mother Emanuel AME Church in Charleston. “Let me be clear for those who don’t seem to know, slavery was the cause of the civil war,” he said.

Trump, too, took a swing at Haley while campaigning in Iowa last week. “I don’t know that it’s going to have an impact, but I’d say slavery is sort of the obvious answer as opposed to about three paragraphs of bullshit. She just talked, nobody knew what she was saying,” Trump told a crowd of supporters in Mason City, Iowa.

But the former president also triggered a controversy of his own when he said during that same Iowa campaign swing that the Civil War “could have been negotiated.”

DeSantis has needled Haley over her answer to the Civil War question. But at the same time he has mocked Haley (she is “sticking her foot in her mouth,” he said on a call with Iowa media Monday afternoon), he’s redirecting the topic toward Trump’s comments — turning the issue into a two-for-one swipe at his biggest rivals.

“He couldn’t even negotiate funding for a border wall when he had a Republican Congress,” DeSantis said on that media call.