Donald Trump now leads polls of GOP voters in Iowa, New Hampshire, South Carolina and nationally. Stopping him in Iowa, a state packed with evangelical voters who may prefer a more devout candidate, is perhaps the surest way to ensure Trump doesn't win the GOP nomination.
But in the final debate before the Iowa caucuses, Trump's rivals all made a risky strategic decision: since the mogul was not on stage, they largely avoided talking about him. Most of the two-hour session was filled with the candidates repeating their talking points and bashing President Obama and Hillary Clinton, along with a few heated exchanges between Texas Sen. Ted Cruz and Florida Sen. Marco Rubio.
There were obvious reasons to attack Trump aggressively. This was the last time that the candidates will speak to a nationally televised audience before the Iowa caucuses.
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Allies of Cruz, seeing Trump move ahead of him in Iowa polls, have started running ads highlighting some of the mogul's past liberal stances on policy issues. New Jersey Gov. Chris Christie, on the eve of the debate, had been suggesting Trump was being disrespectful to Iowa voters by not appearing at the Des Moines debate.
Neither man made these arguments against Trump on Thursday night.
The candidates did not behave like men trailing a front-runner who is looking stronger than ever and appears to be on the brink of wiping them out.
"I'm a maniac and every one on this stage is stupid and fat and ugly, and Ben, you're a terrible surgeon. Now that we've gotten the Donald Trump portion out of the way," Cruz said at the start of the debate, leading to laughter from the audience.
And ex-Florida Gov. Jeb Bush again attacked Trump's proposal to ban Muslims from entering the United States.
But for a full hour of the debate, Trump's name went largely unmentioned. With the network in a feud with Trump, the Fox News moderators had some obvious incentives not to bring up the mogul's name.
But candidates often ignore questions from the moderators to say whatever they want, and it would have been logical for them to blast Trump whenever possible. They did not.
Instead, several candidates suggested Cruz was a flip-flopper and attacked Rubio for his past support of a path to citizenship for undocumented immigrants.
Cruz mentioned that he hoped to appeal to Trump's supporters, and Rubio pledged to unite the Republican Party if he is the nominee. Those were both moves aimed at reaching Trump's voters.
But for now, Trump's voters are with Trump. If the Republican candidates think that Trump skipping a debate will turn Iowans away from supporting him, that is a surprising calculation. Trump has continually broken the rules of modern politics, but his supporters have not broken with him.
If Trump wins the Iowa caucuses on Monday, his rivals may look back on Thursday's debate as a missed opportunity.
This article first appeared on NBCNews.com.