Trump had been publicly mulling the move for a few days, complaining that Fox News host Megyn Kelly, one of three moderators for the event, is biased against him.
Exhibiting the resolute decision-making skills required of a commander-in-chief, Trump asked the Twitterverse to help him choose whether to attend. (Hey, the man loves polls.)
Trump had floated the idea of a debate boycott at least three times before but never followed through, making his latest threat appear dubious at best.
What seemed more plausible was that he would try to use the specter of his absence — and the presumptive ratings drop that would follow — to pressure Fox News into replacing Kelly. We're talking about an ultimatum: Trump on the stage or Kelly at the desk. Your choice, Roger Ailes.
The real estate magnate hinted at playing this game of chicken on Monday.
But Ailes didn't blink. He told The Fix on Tuesday afternoon that if it came down to a choice between Trump or Kelly, he'd pick Kelly.
"Megyn Kelly is an excellent journalist, and the entire network stands behind her," Ailes said in a statement. "She will absolutely be on the debate stage on Thursday night."
But Trump wouldn't back down, either. At a campaign stop in Marshalltown, Iowa, later Tuesday, he initially told the crowd that he "probably won't bother doing the debate." Then his campaign manager, Corey Lewandowski, told The Washington Post that the brash billionaire had made up his mind.
"He's definitely not participating in the Fox News debate," Lewandowski said. "His word is his bond."
Trump's boycott takes his feud with Fox News to a new level. He has bemoaned the cable channel's coverage throughout the race, and the network has occasionally fired back — as it did on Tuesday afternoon before Trump's decision was final and while he was running the online poll.
"We learned from a secret back channel that the Ayatollah and Putin both intend to treat Donald Trump unfairly when they meet with him if he becomes president," Fox said in a tongue-in-cheek statement. "A nefarious source tells us that Trump has his own secret plan to replace the Cabinet with his Twitter followers to see if he should even go to those meetings."
Trump's gripe about Kelly's role in the Fox News debate was different from his complaints in the run-up to previous debates; it was personal, whereas the others were about money and format.
He said he might demand a $10 million appearance fee for a CNN debate in September and $5 million for another CNN debate in December. In both cases, he said the money was for charity; and in both cases, he showed up without receiving a dime. He and fellow Republican candidate Ben Carson also threatened to boycott a CNBC debate in October if the event was not limited to two hours. (CNBC's John Harwood said during the debate that it was going to be a two-hour debate.)
Trump has had it in for Kelly since the first primary debate in August, when she questioned him about disparaging remarks he has made about women with her opening question.
He said the next day that Kelly had "blood coming out of her wherever" when she confronted him — a remark that was widely seen as confirming the very kind of sexism she had asked about in the first place.
It seemed clear what Trump was doing: He was whining about Kelly in advance so that if she got the better of him on Thursday night, he'd have a built-in excuse. He'd be able to say, "See, I knew I wouldn't get a fair shake. What did you expect?"
The Fix's Chris Cillizza, after all, shared a little secret before the last GOP debate a couple of weeks ago: Trump isn't a great debater. Well, here's another little secret: He knows it.
Trump isn't terrible in debates, but he's not as good as he is on the stump, and he's not as good as his top rival, Cruz. Though he declares himself the winner of every debate, his poll numbers tend to dip after these rhetorical clashes.
With less than a week remaining before the Iowa caucuses, Trump appeared be giving himself cover by crying foul before Thursday's debate even began. But when Fox News refused to replace his least-favorite referee, Trump took his ball and went home.