Tuscaloosa, Alabama - Republican presidential hopefuls sought to halt surging Nikki Haley's momentum in an acrimonious fourth primary debate Wednesday, as they made their pitches to be considered the most viable alternative to runaway frontrunner Donald Trump.

The former president – who is seeking to retake the White House despite facing 91 felony charges – maintains a historically large lead and has seen his polling go from strength to strength with each new indictment.

But he has skipped the debates, seeing no advantage in sharing the stage with distant rivals, and rendering them a sideshow to the battle pitting his presidential ambitions against the might of the US justice system.

In Trump's absence, former UN ambassador Nikki Haley touted her rising polling numbers to argue that she had supplanted Florida Governor Ron DeSantis as Trump's understudy, six weeks ahead of the first nomination vote.

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"We have to stop the chaos but you can't defeat Democrat chaos with Republican chaos. And that's what Donald Trump gives us," she said as she pitched a more professional administration than the Trump presidency.

"My approach is different: No drama, no vendettas, no whining."

Only two other candidates qualified for the showdown – tech entrepreneur Vivek Ramaswamy and former New Jersey governor Chris Christie – and a less cluttered stage gave the rivals time to drill into their opponents' policies and records.

Haley's rise has been attributed to strong performances in the previous three debates, and she is riding high on a key endorsement from the powerful Koch family and $250,000 from billionaire Democratic donor Reid Hoffman.

But she went into the debate with a target on her back, as DeSantis and Ramaswamy characterized her as being in the pocket of corporate interests and weak on China, with Ramaswamy repeatedly calling her "corrupt."

Trump (77) is way out front in polling at around 60%, but faces prosecution in four jurisdictions, and will be forced to divide his attention between the election and a series of trials if he wins the nomination.

He is accused of leading a criminal conspiracy to steal the 2020 election, mishandling classified documents, obstructing justice, and lying in business records to cover up hush money payments to a porn actress.

In the battle to usurp Trump should voters get cold feet, DeSantis is averaging 13 percentage points to Haley's 10 in the two major polling averages collated by RealClearPolitics and FiveThirtyEight.

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But Haley has gained the upper hand on DeSantis in some of the early states that matter for building momentum.

Christie has been Trump's loudest critic by far, but is languishing around 2.5% in polling averages.

The ex-governor used his opening remarks to attack his opponents for giving Trump a pass, referring to the former president as Voldemort, the antagonist in the Harry Potter books who "shall not be named."

"This is an angry, bitter man who now wants to be back as president because he wants to exact retribution on anyone who has disagreed with him, anyone who has tried to hold him to account for his own conduct," Christie said.

Ramaswamy enjoyed a surge in donations and headlines thanks to a rambunctious showing in the first debate that ingratiated him to the party's Trumpist base, but is down from a September polling peak.

He was heckled by the audience at the University of Alabama as he echoed Trump's false claims of widespread fraud in the 2020 election and was booed as he attacked Haley on supporting Ukraine against the Russian invasion.

"This is a woman who will send your kids to die so she can buy a bigger house," Ramaswamy said, to even louder boos.

In a rare show of unity, Christie defended Haley and called Ramaswamy "the most obnoxious blowhard in America" as the pair got into a shouting match.

Discussion of foreign policy was dominated by speeches of support for Israel in its response to the deadly October 7 attacks by Hamas militants – and tough talk on China.

"Deterring China's ambitions is the number one national security task that I will do as president, and we will succeed," DeSantis said. "The 21st century needs to be an American century. We cannot let it be a Chinese century."