Republican 2016 presidential candidates are due to take to the debate stage in Iowa later, with the notable exception of frontrunner Donald Trump.
The billionaire businessman will hold a competing rally, which could draw attention from the main event just days before the important Iowa caucuses.
He decided to withdraw after Fox News refused to drop host Megyn Kelly, who Mr Trump says is biased against him.
Voters in Iowa on Monday are due to pick their presidential nominee.
It is seen as the first real test of the election campaign, and the beginning of a series of state-by-state contests to chose delegates for both Republicans and Democrats.
In making his decision to skip the debate in Des Moines, Mr Trump claimed “unfair” treatment from Ms Kelly of Fox News, the conservative US television network that is hosting the debate in Iowa’s state capital, Des Moines.
Friction between the businessman and television host dates back to the first Republican debate, when Ms Kelly challenged Mr Trump over derogatory comments he had previously made about women.
Mr Trump later sparked outrage when he said the Fox News host “had blood coming out of her eyes, blood coming out of her… wherever”. He denied he was referring to menstruation and refused to apologise.
He is expected to hold a simultaneous event – a fundraiser for veterans – which other television networks are likely cover.
In polls, Mr Trump is currently leading the Republican pack in Iowa with 33.2% of the vote (6.5 percentage points above his nearest rival, Texas Senator Ted Cruz), and is in the lead nationally with 36.2% (16.8 points over Mr Cruz).
The rival event could steal voters’ attention away from the main event, where other candidates are desperate to stand out ahead of Monday’s vote in Iowa.
Analysis – Anthony Zurcher, BBC North America reporter in Iowa
It’s like a Mad Max film without Max; Jurassic Park without the dinosaurs. But it now appears Donald Trump will be a no-show for Thursday night’s Republican debate.
His absence presents a particular challenge to Ted Cruz. The Texas senator traded barbs with the New Yorker at the last debate, although neither gained the upper hand. Since then, he has been sharpening his attacks and appeared eager to use his polished debate skills to win round two.
Mr Trump’s absence denies him that opportunity. Mr Cruz is likely to try to paint his opponent, in absentia, as either a coward afraid of a fight or an intemperate personality given to rash decisions.
Pummelling an empty lectern can grow tedious, however, and Mr Cruz – who is knotted with Mr Trump atop Iowa polls – may draw fire from the rest of the field.
Meanwhile, Mr Trump will take his spotlight to a different venue, where he can employ his unconventional rhetorical skills without challenge.
So far every move Mr Trump has made, no matter how seemingly risky, has ended up with him on top. If he loses Monday’s caucuses, however, his debate decision will be heavily criticised – and could dent his reputation as a political savant.
Iowa is the first state to vote in the primary election process, and can be an important moment for candidates to gain momentum as they try to secure their party’s nomination.
Those on stage looking to pique voters’ interest will be:
- Texas Senator Ted Cruz, who will look to close the gap between himself and Mr Trump in both Iowa and national polls
- Florida Senator Marco Rubio, who is trying to frame himself as the moderate alternative to Mr Trump and Mr Cruz
- Retired neurosurgeon Ben Carson, whose campaign has flagged in recent months, but who has loyal support in Iowa
- Former Florida Governor Jeb Bush, who is looking for any chance to revive his campaign amid calls to drop out of the race
- New Jersey Governor Chris Christie, who is looking to build support ahead of the New Hampshire primary – which he is targeting heavily – on 9 February
- Ohio Governor John Kasich, who, like Mr Christie, is looking ahead to the New Hampshire primary, where much of his campaign’s life is staked
- Kentucky Senator Rand Paul, who will be on the main stage for the first time after being relegated to the so-called undercard debate previously
The BBC will be providing live updates online of all the events, including the latest from Iowa as residents prepare to officially begin the 2016 election season.