•Both candidates will likely have to exceed expectations to step out from under Mr Trump's shadow and keep their campaigns alive.
•The 15 January caucus will be the candidates' first electoral test in their effort to become the Republican presidential nominee in 2024.
Florida Governor Ron DeSantis will face off against former UN ambassador Nikki Haley in the last Republican debate before the Iowa caucus next week.
The debate in Des Moines comes as both candidates lag far behind former President Donald Trump in the polls.
The 15 January caucus will be the candidates' first electoral test in their effort to become the Republican presidential nominee in 2024.
Mr Trump will not attend the debate but will appear in a Fox News town hall.
He has so far boycotted every Republican debate and holds commanding poll leads in Iowa ahead of the caucuses.
The CNN debate, which is scheduled for 21:00 EST (02:00 GMT), offers Mr DeSantis and Ms Haley, who stand in second and third place in the polls respectively, a final opportunity to make their case to Iowans ahead of next week.
Two other candidates, entrepreneur Vivek Ramaswamy and New Jersey Governor Chris Christie, failed to qualify. Mr Christie announced he was dropping out of the race on Wednesday just hours before the debate.
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Meanwhile, there was last-minute sparring between Ms Haley and Mr DeSantis over the airwaves, with a pro-Haley ad criticising the Florida governor for allegedly mischaracterising her comments on Gaza, immigration and Hillary Clinton. The ad was entitled "DeSantis is lying because he's losing".
A super PAC that supports Mr DeSantis also released an ad in Iowa which attacked Ms Haley for saying that the upcoming New Hampshire primary - the second Republican contest - would "correct" the results of Iowa.
"Nikki Haley told New Hampshire what she really thinks about Iowa," the narrator says in the video.
Ms Haley was recently buoyed by fresh polling that suggests she has cut Mr Trump's lead in New Hampshire to single digits. Analysts have said a strong showing there could make her Republican party's prime alternative pick to the former president.
Since serving as Mr Trump's ambassador to the UN during his time in the White House, she has broken with her one-time boss on key areas of foreign policy and criticised his willingness to do business with China.
She has seemed to promise a return to a more traditional Republican party, embracing a hawkish foreign policy and faulting members of both parties, including Mr Trump, for running up the national debt.
A former South Carolina governor, Ms Haley has also argued that she is a more palatable candidate for undecided voters than Mr Trump.
But as her star has ascended in recent weeks, she has faced increased attacks from Mr Trump and Mr DeSantis.
She was also criticised after she failed to say that the Civil War was fought over the issue of slavery.
By contrast, Mr DeSantis, the two term Florida governor, has sought to tack hard to Mr Trump's right.
He has fought a campaign based broadly on culture wars issues and has touted his record of tackling what he called "woke" policies in his home state.
While an early favourite as an alternative choice to Mr Trump, he has struggled to differentiate himself from his one-time ally. The former president raised Mr DeSantis from relative obscurity as a member of Congress and endorsed him in his run for the Florida governor's office in 2017.
It does appear that Mr DeSantis is better suited to perform well in Iowa than Ms Haley, where evangelical voters, a key voting block in the state, could be attracted to his staunch conservative values.
Nevertheless, both candidates will likely have to exceed expectations to step out from under Mr Trump's shadow and keep their campaigns alive.