US presidential hopefuls limbered up Wednesday for the fourth debate in the 2024 Republican primary, with the race narrowing to a head-to-head battle to be the main alternative to runaway frontrunner Donald Trump.

The former president -- who is running 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.

Meanwhile former UN ambassador Nikki Haley has been on an upward trajectory, threatening Florida Governor Ron DeSantis's status as the most viable Trump understudy, six weeks ahead of the first nomination vote.

She hit out at DeSantis's faltering polling numbers and Trump's foreign policy record on Friday, telling Fox News she was proud of creating jobs and curbing crime as South Carolina governor in the 2010s.

"Our focus is on making America strong and proud. We're going to do it by getting our economy back on track. We're going to get our kids reading again," she said.

"We're going to make sure that crime comes down. We're going to close our borders once and for all, and we're going to let the world know America's back."

Only four candidates from the last debate are still in the race and expected to attend Wednesday's showdown in Alabama: DeSantis, Haley, entrepreneur Vivek Ramaswamy and former New Jersey governor Chris Christie.

- Upper hand -


Much of Haley's momentum has been credited 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 LinkedIn co-founder Reid Hoffman, a major Democratic donor.

In a sign of the Desantis camp's eagerness to gain back ground from Haley, spokesman Bryan Griffin slammed the donation as evidence she is actually "a liberal."

"She would let corporations set immigration policy, roll out the red carpet for China, hike taxes on hardworking Americans, and require social media users to register with the government," he argued.

Trump, 77, is way out front in polling at around 60 percent, 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 racketeering and leading a criminal conspiracy in his efforts to overturn the results of the 2020 election, mishandling classified documents 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.

But presidential nominees aren't selected in national primaries and in some of the early states that matter most, Haley has the upper hand on DeSantis in the non-Trump undercard.

Though DeSantis has been holding second place in Iowa -- the all-important first nominating contest -- Haley has the runner-up spot in New Hampshire and her home state of South Carolina.

Ramaswamy enjoyed an upsurge 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.

With most candidates still tip-toeing around Trump, Christie has been the former president's loudest critic by far -- but he is languishing around 2.5 percent in polling averages.