The Republican field has shrunk to just four candidates for its final presidential primary debate on Wednesday in Alabama.

Ron DeSantis, Nikki Haley, Chris Christie and Vivek Ramaswamy were the only candidates who qualified to appear in the last bout, the party said.

The debate, the fourth of the primary race, required candidates to have 80,000 donors and 6% in two US polls.

Republican frontrunner Donald Trump is once again skipping the forum.

With the field narrowing, and only weeks to go until the early-voting state of Iowa casts its first ballots on 15 January, pressure is building on candidates to show they are capable of taking on Mr Trump for the Republican nomination.

The debate will be hosted by NewsNation at 20:00 local time (01: 00 GMT) and is taking place at the University of Alabama in Tuscaloosa. It will be moderated by a panel of conservative-leaning journalists - including former Fox News host Megyn Kelly.

The qualifications are the strictest of the debates so far. Earlier on Monday, North Dakota Governor Doug Burgum dropped out of the race as it appeared unlikely he would be allowed to participate on the debate stage.

Mr Burgum, who had struggled to make his mark in previous debates, hit out at the process in his statement, saying the thresholds set by party leaders were "taking the power of democracy away from the engaged, thoughtful citizens of Iowa and New Hampshire".

Former Arkansas Governor Asa Hutchinson - an outspoken critic of Mr Trump - remains in the race, but also missed out on a spot on the stage. He is achieving around 1% support in national polls.

"The fourth debate is another fantastic opportunity for our Republican candidates to share our winning agenda with the American people," party chairwoman Ronna McDaniel in a statement on Monday night.

She noted that former Republican President Ronald Reagan visited the same campus during his successful re-election campaign in 1984 "just before cruising to a landslide victory".

The third debate in Miami, hosted by NBC, featured heated exchanges between biotech millionaire Mr Ramaswamy, former UN Ambassador Haley and Florida Governor DeSantis.

Eyes will likely be on Mrs Haley, who recently received the support of the wealthy Koch network, to see if she can capitalise on their backing.

Meanwhile, calls are growing for former New Jersey Governor Christie to exit the race and throw his support to another candidate.

"The people who are supporting Chris are not supporting him because they love Chris Christie - they want someone to take on Trump," former Republican senator and presidential candidate Rick Santorum told the New York Times on Monday.

Mr DeSantis, whose campaign has reportedly been engulfed by infighting, has also faced calls to drop out of the race and allow Ms Haley to try to unify an anti-Trump bloc.

But the Florida governor insisted on Sunday that he could win the Iowa caucus.

Mr Trump, who has skipped all the debates thus far, will instead appear at a Fox News town hall hosted by Sean Hannity on Tuesday night and hold a private fundraiser on Wednesday.

The former president, who maintains an overwhelming led in the polls, has repeatedly urged the party to cancel the debates, arguing that no candidate can catch him.